Dockerizing Jenkins 2, part 3: Securing password with docker-compose, docker-secret and jenkins credentials plugin. Setup a private space for you and your coworkers to ask questions and share information. Note: You will have problems if you start the PostgreSQL container with a Bind Mount on a Windows host. This is what you use when you have an application made up of multiple microservices, databases and other dependencies. 2 you can use long syntax which allows the configuration of additional fields that can be expressed in the short form such as mount type (volume, bind or tmpfs) and read_only. The target path inside the container # should match what your application expects. Step 2 - Install Docker-Compose. Docker Compose. I’ve been running into a lot of issues with getting my docker-compose. Creating a Docker Swarm. (See also Use Bind. Also, there is a Dockerfile file in the web application. yml to the server. I was having this issue (at least I think it's the same issue), but since I wanted to keep Hyper-V off in order to use accelerated emulation for Intel + Android, I couldn't use the Docker for Windows GUI app and look for those Shared Drives settings that @shin-mentioned, because I was using docker-machine. It has been asked on #docker-dev recently if it was possible to attach a volume to a container after it was started. target: C:\app For this, I needed to use 3. The “wait for the master” logic is going to come in very handy when we wrap the master and slave into a docker-compose file. The -e CASSANDRA_BROADCAST_ADDRESS=10. The containers, when activated, use the Linux kernel namespaces and are operating isolated from the rest of your server, except for storage/mount points and networking, depending on the configuration of the container. Let's start with the project structure, we have a docker-compose. The Compose file defines the settings needed to run your application. The keys to getting this successfully deployed to local Kubernetes were in modifying docker-compose. The following three commands show you how to verify that all of these components installed successfully and find out which versions you have. Original post is here. crt files in that directory or bind-mount your own SSL directory to /Kiwi/ssl inside the docker container!. This comment has been minimized. For example, in our stephenafamo/adonisjs image, a new application will be created in the /var/www directory if an empty volume is mounted there. For example, you can run a web server, backend database, and your application code as separate services. What you should notice here is that you are binding port 80 and 443. Docker Compose. Docker compose tutorial for beginners by example [all. For this example, let’s call this image binfalse/php-fpm-extended. yml Docker Swarm Visualizer The Docker Swarm Visualizer image connects to the Docker socket and shows a really nice visualization showing all of the nodes in your Docker cluster (or just one on your development machine) and all of the containers running on it. Docker-in-Docker works well, and is the recommended configuration, but it is not without its own challenges: When using docker-in-docker, each job is in a clean environment without the past history. C reating a development environment may seem like a trivial task for many developers. Docker in Action, Second Edition teaches you the skills and knowledge you need to create, deploy, and manage applications hosted in Docker containers. yml allows you to configure all those services in one place and start them. An easy way to setup a basic DNS server with Docker is to use the BIND DNS server bundled with the Webmin interface. Mount in Docker. The base image is centos:7. Solution 2. Docker swarm operates with a recipe, telling it which containers to spin up, which ports to publish, which volumes to mount, et cetera. Docker installed with the instructions from Step 1 of How To Install and Use Docker Compose on Ubuntu 14. We will only create two services db database PostgreSQL and web is the rails application itself. yml, The second line creates a bind mount, which will always. Also, there is a Dockerfile file in the web application. By default, in production mode the tool uses PostgreSQL as database, Redis for caching. com throughout. Step 3 - Create and Configure the Docker Environment. 3 (10 ratings) Course Ratings are calculated from individual students’ ratings and a variety of other signals, like age of rating and reliability, to ensure that they reflect course quality fairly and accurately. How To Restore a Missing Redis Service. yml file in a directory called tick. New users cloning the repo and clone it anywhere in their host system, and run docker-compose up -d and get pretty much the same result. Docker assigned a unique id under /var/lib/docker/volumes that was not easily managed by sight. View docker-compose. How to write docker-compose. Docker and Magento. Docker-in-Docker works well, and is the recommended configuration, but it is not without its own challenges: When using docker-in-docker, each job is in a clean environment without the past history. After that a docker run --rm hello-world should yield exactly the same results as it did in a Docker Toolbox terminal. A definition file describes the inter-dependencies between the services, including details such network or storage requirements, in a manner that is easy to maintain. And that's part of the problem. This post uses an adaptation of jpetazzo’s technique in Attach a volume to a container while it is running from 2015. The db service also uses a named volume called dbdata (second path under db service volumes ), but defines it using the old string format for mounting a named volume. This quick post demonstrates using Percona Server for MySQL in Docker Swarm with some new authentication provisioning practices. Instead of specifying one single path (as we did when defining volumes) we will specified 2 paths separated by a column. Otherwise, you’ll run Docker inside Docker (DinD) and that not a very good idea. Note: Verdaccio runs as a non-root user (uid=100, gid=101) inside the container, if you use bind mount to override default, you need to make sure the mount directory is assigned to the right user. Building Docker images with GitLab CI/CD GitLab CI/CD allows you to use Docker Engine to build and test docker-based projects. There are several backend storage supported by Vault. Getting Started Binding Ports Binding Ports. AWS Documentation » Amazon ECS » Developer Guide » Amazon ECS Task Definitions » Task Definition Parameters Task Definition Parameters Task definitions are split into separate parts: the task family, the IAM task role, the network mode, container definitions, volumes, task placement constraints, and launch types. Why I don't use Docker much anymore 11 Mar 2015 17 minute read Update: The conclusion of this post talks about managing Docker kittens with Ansible; I no longer do that, but instead use Amazon ECS with Terraform. mount -o bind /data /var/lib/docker Automatic cleanup. The Docker Compose file calls for three Docker containers (GitLab, Postgres, Redis) to be run and linked together utilizing their upstream official Docker images. If networks parameter is provided, will inspect each network to see if there exists a bridge network with optional parameter com. Automation Step by Step Docker Compose (in Hindi) : Bind Mount - Duration: 3:21. 04 Note: Even though the Prerequisites give instructions for installing Docker on Ubuntu 14. Docker (01) Install Docker (02) Add Container images (03) Access to Container Services (04) Use Dockerfile (05) Use Persistent Storage (06) Use Docker-Registry (07) Use Docker-Registry (Auth) (08) Use Docker Compose (09) Configure Docker Swarm; Kubernetes (01) Minikube : Install (02) Minikube : Deploy Pods (03) Kubeadm : Install (04) Kubeadm. Docker collaborates with the open source ecosystem through an array of projects to fuel the containerization movement, the Docker platform and other Docker products. with README and docker-compose example in repository. Writing Dockerfiles for Windows containers. yml file (in the same directory), allowing you to add more services, or for example to change where the volumes are stored (see official documentation). The changes include a separate top level key named volumes. Jenkins then needs to build a new docker image and deploy it using docker-compose. Let Docker for Windows restart and you’re good to go. yml file in our example Git repo for running GitLab, which we will utilize for this example. Traefik is a reverse proxy that will handle all web traffic and automatically retrieve Let’s Encrypt SSL certificates. Then they can start the multi-container services with a single command. With the updates to docker-compose in v3. The base image is centos:7. Earlier, one has bind mount it with the -v option and that had to be done with the --privileged option. 2 as the docker compose yaml version. env file which simply defines key-value pairs evaluated by docker-compose. Docker and Magento. It provides us Docker basic commands and system and It's easy to understand. Where multiple options are present, separate them by commas. In this article we cover how to implement DNS server using docker container. Once you have the dependencies installed, create a Docker Compose file called docker-compose. This ensures that the tests are run against every new commit to Github. From Development to Production Hi, I'm Nick. In this first post, of which many will follow, we show you how to set up Docker for Magento 2 development. If the challenge is not known by other Træfik instances, the validation will fail. exe create default. docker-compose. If the socket is mounted, it does not matter whether you run as a user that is in the docker group or not. $ docker-compose -f docker-compose. 13 is the ability to use compose files to deploy swarm mode services!. It consists of a web UI that allows you to easily manage your Docker containers, images, networks and volumes. Getting Started Binding Ports Binding Ports. In this config you will learn how to link two or more docker containers together using a simple docker networking technique. Using bind-mount volumes is host-dependent. Allow a user to use Docker. cfg file, you can use HAProxy's graceful reload feature by sending a SIGHUP to the container: # docker kill -s HUP haproxy Docker-compose. Docker Machine installs and configures Docker hosts on local or remote resources. See the Developing inside a Container article for additional information. The Text Recognizer {COMPUTER_VISION_API_KEY} must be the same for both the formrecognizer and ocr containers. yml inside iris_community. GitHub Gist: instantly share code, notes, and snippets. The keys to getting this successfully deployed to local Kubernetes were in modifying docker-compose. After that a docker run --rm hello-world should yield exactly the same results as it did in a Docker Toolbox terminal. In the examples, we're spinning up a new docker container, so we're always using the up command. All Prometheus services are available as Docker images on Quay. Scott provides the fedora-dockerfiles package in docker with lots of “Dockerfile” examples. This tutorial will use example. Also, there is a Dockerfile file in the web application. Setup Using Docker. The extra line command: echo is there so that, when Docker Compose starts the service, it terminates immediately. By default, the Docker engine interacts with DockerHub , Docker's public registry instance. SSH public key authentication is a convenient, high security authentication method that combines a local "private" key with a "public" key that you associate with your user account on an SSH host. The following three commands show you how to verify that all of these components installed successfully and find out which versions you have. In docker, there are two concepts for docker volume. Use of shared volumes has the benefit of being host-independent. Setup a private space for you and your coworkers to ask questions and share information. The Docker Compose file calls for three Docker containers (GitLab, Postgres, Redis) to be run and linked together utilizing their upstream official Docker images. :/app in a docker-compose. The commands used throughtout this video are below. Docker assigned a unique id under /var/lib/docker/volumes that was not easily managed by sight. yml, so that the DOCKER_JUPYTER_IMAGE env variable matches this one. If a readonly property is is defined and true, the volume will be mounted in read-only mode. BIND is an open source software that implements the Domain Name System (DNS) protocols for the Internet. We will run the same previous application of wordpress and MySQL linking in this tutorial but with Docker compose which was launched recently and is a tool use to define and run complex linked applications with docker. We also pass the name of the model as an environment variable, which will be important when we query the model. Since Docker day zero we’ve dreamt about 15 second project ramp-up times, versioned development. Stop the application, either by running docker-compose down from within your project directory in the second terminal, or by hitting CTRL+C in the original terminal where you started the app. You may quickly notice that bind-mount does not work correctly. yml file in your project] docker-compose up [run docker compose in a detach mode (background)] docker-compose up -d [run specific service from your yml file] docker-compose up service_name. This is a more complex diagram than others so that interrelationships can be illustrated. High Availability. Response JSON Object: logs (string) – logs string; tail (int) – the last N line of output. In thtis article, I would like to share basic knowledge about Docker volume. But introduces the problem of docker-compose not being very good at cleaning up after itself. With little to no modification it even can use compose YAML files. Working with docker-compose When using docker-compose for development you can create custom docker network and use it as domain names source. This section will walk you through how to generate these keys and add them to a host. For this example we’ll place our docker-compose. This article is part of a series: Docker — from the beginning part I, this covers why Docker, the basic concepts and the commands we need to manage things like images and containers. The Compose file is a YAML file defining services,networks and volumes. On this page. Compose gave us some one-point management tools quite useful. docker volume create --opt type=none --opt device= --opt o=bind TL;DR of the rest: from/host:to/container actually is shorthand for a mount command with bind option, meaning even though docker has merged volumes and binds together syntaxically you actually need to create a bind mount, not a volume. Docker Compose is a tool for building and running applications composed of multiple Docker containers. Quickstart: Compose and WordPress. In this article, I will write about "data mount (volume), Docker Network, Docker Compose". yml in your project directory to add a bind mount for the web service:. Deploy Swarm Services with the new docker `stack` and a compose file! 26 January 2017 On of the new additions of Docker 1. vim docker-compose. yml so they can be run together in an isolated environment. In this case, we are using docker-compose run to start a certbot container and to override the command provided in our service definition with another: the renew subcommand. I’ve been running into a lot of issues with getting my docker-compose. yml is configured witha default self-signed certificate stored in etc/kiwitcms/ssl/. This ensures that the tests are run against every new commit to Github. docker volume create --opt type=none --opt device= --opt o=bind TL;DR of the rest: from/host:to/container actually is shorthand for a mount command with bind option, meaning even though docker has merged volumes and binds together syntaxically you actually need to create a bind mount, not a volume. While nice and simple, this approach has a major security hole – a simple “docker inspect” command would expose passwords or tokens configured in clear text via environment variables. Setup Using Docker. Docker Compose. When you use a bind mount, a file or directory on the host machine is mounted into a container. Step 2 - Install Docker-Compose. The default path for a Compose file is. If you found the technical concepts in this post helpful, you may want to keep the Outrigger documentation on Working with Volumes and Changing Container Configuration handy as. -t react-cli. Docker Swarm mode introduces a concept which is the clustering of Docker nodes. The problem was, the scaffold that were generated by cli were on docker container, not in our development machine. View docker-compose. Query Parameters: tail – (optional) Output the specified number of lines at the end of logs. They contain open source. This post continues where previously How to install docker on Alpine Linux VM left, where we deployed an Alpine Linux virtual machine in a Proxmox host, created a docker user and installed docker engine. Clustering / High Availability on Docker Swarm with Consul ¶. This post uses an adaptation of jpetazzo’s technique in Attach a volume to a container while it is running from 2015. If you have some update to do, update the initializer service and re-deploy it. debug[ ``` ``` These slides have been built from commit: ead027a [share. The Compose file defines the settings needed to run your application. I then placed my compose file, named docker-compose. yml documentation for details. See the Developing inside a Container article for additional information. -t react-cli. yml file (in the same directory), allowing you to add more services, or for example to change where the volumes are stored (see official documentation). Prerequisites Install Docker for Desktop Docker has great tutorials for beginners as well as advanced users. Docker is an open-source project to easily create lightweight, portable, self-sufficient containers from any application. -f docker-compose. Or if you already have a docker-compose. NET Core projects. with README and docker-compose example in repository. Supermarket belongs to the community. docker – See how to install docker here. The up command is for scaffolding the whole environment. Docker build step plugin. That includes an MQTT broker as the source, Zookeeper, one Kafka broker as well. /src" through volumes. ### Format:. For example, you can run a web server, backend database, and your application code as separate services. A good strategy is to grant group access to gid 1000 or 0 for the local directory. In this tutorial, we’ll use Kafka connectors to build a more “real world” example. How to create a docker-based LAMP stack using docker-compose on Ubuntu 18. Docker has stabilized a lot since my early experiences as well. We will run it as a multi-container app and orchestrate it using Docker Compose. For an example of using local-persist in docker-compose. But introduces the problem of docker-compose not being very good at cleaning up after itself. If you are not familiar with these tools, I will introduce them to give you a brief introduction. yml works, but I would like to have the mount point relative to the yml folder, like. Docker Compose installed on your server, following Step 1 of How To Install Docker Compose on Ubuntu 18. With docker-compose we were able to deploy an application and scale up and down the number of instances of each application block (service). yml, change all images with the ones in docker/docker-compose. This course covers basics of Docker, Different ways to install docker, working with Apache & Nginx web server images, working with database images, persistent storage, bind mounts and volumes, docker networking, using dockerfile, Deploying multi container applications using Docker Compose, Docker Swarm Cluster. Docker provides packages that configure the Docker environment on macOS, Windows, and Linux. yml so they can be run together in an isolated environment. This post uses an adaptation of jpetazzo’s technique in Attach a volume to a container while it is running from 2015. First, we stop and kill the old container from the run command, and then use the docker compose command to run this file:. New users cloning the repo and clone it anywhere in their host system, and run docker-compose up -d and get pretty much the same result. Otherwise though without the ADD this looks correct in docker-compose, you mount your application source into the container and assuming you're working with an interpreted language changes you're making should be visible (I don't know enough about Ruby to say if there's anything going on there). In the following example, web is stopped before db and redis. target: C:\app For this, I needed to use 3. Step 6 - Testing Reference. The same container that a developer builds and tests on a laptop can run at scale, in production, on VMs, bare metal, OpenStack clusters, public clouds and more. Firstly is about bind mount: mount on host storage. When Træfik generates a new certificate, it configures a challenge and once Let's Encrypt will verify the ownership of the domain, it will ping back the challenge. However, each example also exists in its standalone file as shown below: However, each example also exists in its standalone file as shown below:. This is commonly referred to as a bind-mount within docker terminology (though technically speaking, all volumes are bind-mounts in the sense of what is actually happening). This is the reason I am not using docker-compose. For example, I’m running the Azure CLI both under WSL and under Windows, but I’ve symlinked docker in WSL to point to docker. If the file or directory you're trying to bind-mount does not exist, docker has no way to determine if you're expecting a file or a directory, in that case (if /usr/src/app/app. Instead of specifying one single path (as we did when defining volumes) we will specified 2 paths separated by a column. Docker Document - docs. Step 5: Edit the Compose file to add a bind mount. Such volumes are not "managed" by Docker as per the previous examples -- they will not appear in the output of docker volume ls and will never be deleted by the Docker daemon. Then, instead of running your application directly with Docker, you can simply run it with docker-compose up. Traefik needs somewhere to store SSL certificates, so we’ll create a file that Docker Compose will bind mount into the container. With the updates to docker-compose in v3. The “wait for the master” logic is going to come in very handy when we wrap the master and slave into a docker-compose file. In this part, we shall take a look at Docker Volumes. and pros of using docker compose not apparent, but if you have 5+ containers it is too hard to run them together without compose. Docker is a popular containerization tool used to provide software applications with a filesystem that contains everything they need to run. Host Volume: what you refer to as a mount in a container, the more common term is a bind mount. Step 4 - Create Docker Compose File. Define the services that make up your app in docker-compose. 04, the docker commands for Docker data volumes in this article should work on other operating system as long as Docker is install. For this example, we’ll be using a host by the name of swarm-01 as a node manager. In this post we take a look at how to setup and configure Docker Secrets while in Docker Swarm mode. Learn more about Teams. json file in your project. This is necessary because Docker containers do not have access to host data on purpose. 7 and Docker Toolbox for windows version 18. Demo Scenarios InfluxDB and Zipkin Scenario. Named Volume: any volume managed by docker which you give a name. Step 1 - Install Docker. It means how can I define bind mount and managed mount when using docker-compose. If you're running Windows or Mac, Docker Compose is already installed as it comes in the Docker Toolbox. Best practice: minimal, read-only mounts $ docker run it --rm -v /subdir/we/need:/dir:ro alpine sh 48. Docker on Windows uses VM for Linux based docker containers. yaml I have two services being setup, one is a MySQL instance and the other is my custom Django App. Deploying and serving your Java application Before we get any further I want to clarify something that puzzled me a lot when I started using Tomcat and that is the app deployment. Docker images for Kibana are available from the Elastic Docker registry. Docker Engine is the industry’s de facto container runtime that runs on various Linux (CentOS, Debian, Fedora, Oracle Linux, RHEL, SUSE, and Ubuntu) and Windows Server operating systems. 42 is setting an environement variable that will be passed to the container being created by the image here. In this tutorial, we’ll use Kafka connectors to build a more “real world” example. We give the following guidance, broken down by launch type. We will only create two services db database PostgreSQL and web is the rails application itself. Skip to end of metadata. docker – See how to install docker here. You may quickly notice that bind-mount does not work correctly. The VOLUME directive creates a mount point with the specified name and marks it as holding externally mounted volumes from native host or other containers. What are bind mounts You can remove unused volumes using docker volume prune. ## ${VARIABLE:-default} will evaluate to default if VARIABLE is unset or empty in the environment. If the file or directory you're trying to bind-mount does not exist, docker has no way to determine if you're expecting a file or a directory, in that case (if /usr/src/app/app. For this example we’ll place our docker-compose. host_binding_ipv4. Connect to Testnet by runing iServer with updated config:. The cio volume create command creates a virtual disk (vdisk) as a Linux block devices in /dev/vdisk and works with standard Linux. For an example of using local-persist in docker-compose. In this case, we are using docker-compose run to start a certbot container and to override the command provided in our service definition with another: the renew subcommand. yml is the default file that will be used. The depends_on: tag in docker-compose doesn’t serve as well, as the jenkins master takes more time to be fully up and running than what docker-compose estimates it to be. By creating a container specifically for mounting volumes to, this created an organizational structure whereby the data container was named and easily referenced. Since a Docker volume (or bind mount) can be attached to multiple containers, it may be useful for only some of the containers to be able to write to it. I have a question how to proceed if I want to test a new binding in a docker container environment. lock was built into the image, but it wasn't reflected back to your Docker host's file system because there's no concept of a volume mount at build time. Docker Engine Sparked the Containerization Movement. A list of all published Docker images and tags is available at www. High Availability. The volume driver will be the name of the storage driver, which is the name of the stack. However, there are additional ways to use the docker-compose files that are worth exploring in further detail. Docker Secrets and MySQL Password Management August 14, 2017 Announcements Trond Humborstad In this posting we will look at currently recommended ways of managing passwords in MySQL Docker containers and explore whether the recently introduced concept of Docker Secrets could play a role in this area. By default, the Docker engine interacts with DockerHub , Docker's public registry instance. Updated on April 19th, 2019 in #dev-environment, #docker. This web page documents how to use the sebp/elk Docker image, which provides a convenient centralised log server and log management web interface, by packaging Elasticsearch, Logstash, and Kibana, collectively known as ELK. Volumes have a source and a target. Toggle navigation. 7 (6 ratings) Course Ratings are calculated from individual students’ ratings and a variety of other signals, like age of rating and reliability, to ensure that they reflect course quality fairly and accurately. # install Docker Compose build dependencies for Ubuntu 18. Also that (a) absolute paths require a drive letter and (b) you can't bind mount over a non empty directory inside the container on Windows, which is a big difference. Docker known issues. My compose file will create a bind mount using this directory and I can use it to pass data and code between my Windows file system and my container. The last item we will talk about is named bind-mount and consist of mounting a host's folder into a container's folder. The Visual Studio Code Remote - Containers extension lets you use a Docker container as a full-featured development environment. ### Useful if your host computer run other Docker services that you want to connect to or reach ### from within the Devilbox network by a custom hostname. Docker volumes default to mount in read-write mode, but you can set it to be mounted read-only by adding the :ro at the end of the volume. If I understand correctly, you want to create a bind mount for mariadb so that a directory in your host is mounted inside the container. Docker compose example. We can use the yaml files to configure the application services. You can mount a host path as part of a definition for a single service, and there is no need to define it in the top level volumes key. -f docker-compose. The Compose file defines the settings needed to run your application. jetty,docker,dockerfile. Each service can be scaled by adding more containers if necessary. I wasn't aware of this feature until Bret Fisher told me, so it's quite possible many others don't know too! As of Docker Compose 1. yml to mount a local directory. This can be shared amongst other containers. Hope I can make it work). The next thing would be to deploy a stack (a complete web application, with backend, frontend, database, etc) using this Docker Swarm mode cluster. Cluster-aware means that you can query for a list of services or inspect a task inside any node on the cluster, as long as you’re executing the Docker API request on a manager node. Q&A for Work. WHITE PAPER | INTRODUCTION TO CONTAINER SECURITY WHITE PAPER / 5 Docker container technology increases the default security by creating isolation layers between applications and between the application and host and reducing the host surface area which protects both the host and the co-located containers by restricting access to the host. For example the following will change the volume for postgres to be stored in your home directory:. yml logs -f. This prevents port collisions with already-open ports. If the diagnostic is good, your are now ready to build your real-time Rest Api. Please note the following instructions use the latest Logagent image from Docker Hub. The next thing would be to deploy a stack (a complete web application, with backend, frontend, database, etc) using this Docker Swarm mode cluster. 1:8001 --workers=3" The problem is that GUNICORN_CMD_ARGS is not processed correctly somewhere in the. target: C:\app For this, I needed to use 3. yml # Docker compose for sonarquebe with volumes mapped from host (you are telling docker which path to mount) and plus restart always to achieve autostart if docker containers fail or stop. Portainer is comprised of two elements, the Portainer Server, and the Portainer Agent. The docker-compose. Say for example you want to mount a "db" directory in your filesystem (I will assume it's in the same folder where the docker-compose. Note: Verdaccio runs as a non-root user (uid=10001) inside the container, if you use bind mount to override default, you need to make sure the mount directory is assigned to the right user. If you are bind-mounting a local directory or file, ensure it is readable by this user, while the data and log dirs additionally require write access. exe installed with Docker for Desktop (i. The command we will be executing is the docker command with the swarm init options. Configuring centralized logging from Docker To send logs from applications running in a Docker container, choose based on your Docker version and deployment preferences. crt files in that directory or bind-mount your own SSL directory to /Kiwi/ssl inside the docker container!. docker build -t my_mongodb. Quickstart: Docker Compose and WordPress You can use Docker Compose to easily run WordPress in an isolated environment built with Docker containers. PostgreSQL. Building custom Docker images. Docker images are assembled from versioned layers so that only the layers missing on a server need to be downloaded.