Docker by example

When I started learning Docker I didn't find a short step by step guide so I decided to make my own. Although I describe a specific use case, I think these steps can easily be transferred to other use cases.

The use case

Prepare a project specific development environment that can be fast and easily installed on developer machines. I'm using a docker image for this since it can not only provide the IDE but also the complete tool development chain such as JDKs and build tools like ant or gradle.

Terms

First of all its important to understand the difference between containers and images.

image

Think of an image as a template for creating containers. It contains a filesystem and some definitions (e.g. maintainer and the ports to export etc.). An image can be based on another image and can itself be the basis for other images.

container

A container is an instance of an image. Running an image creates a new container based on that image.

Dockerhub

Search Dockerhub for an image that can be used as a base for the container.

Pull the image from dockerhub

sudo docker pull joemat/docker-eclipse-for-rcp

This step is optional: When calling docker run with an image that isn't available locally the image is pulled automatically.

Create and run a container based on this image

xhost +local; sudo docker run --name eclipse_luna -it -v /home/joemat/development:/share/development -e DISPLAY -v /tmp/.X11-unix:/tmp/.X11-unix joemat/docker-eclipse-for-rcp

• The --name parameter assigns a name to the container that can be used to identify it in subsequent commands.
• The xhost +local command and the -e DISPLAY -v /tmp/.X11-unix:/tmp/.X11-unix parameters are needed to enable the container to create X windows (see Fábio Rehms Blog).
• -v /home/joemat/development:/share/development "mounts" the local directory /home/joemat/development as /share/development in the container.
• The container is shut down when the application (eclipse) terminates.

Now it's time to adjust the application to our needs - e.g. create and configure Eclipse workspaces, install additional software etc.
These docker commands may be useful at this point:

• sudo docker start -i eclipse_luna
  
  Restarts the docker container when it is stopped (e.g. after eclipse terminated)
  • No need to pass the -v and -e parameters again.
• sudo docker exec -ti eclipse_luna /bin/bash
  
  Executes a different application (a shell in this case) in the running container. This could e.g. be used install additional software packages.

Create an image from the current container status

When all configuration and adjustment has been done, it's time to save the current state of the container. This is done using the docker commit command, which creates an image from a container. (The container should be stopped before running docker commit.)

sudo docker commit -m "eclipse for project foo" eclipse_luna eclipse_for_foo

Share the image

Now it's possible the push the image to the docker hub using the docker push command. Alternatively you can push it to your local docker repository. (The Howto from Nik van der Ploeg describes the installation of a private registry server.) Pushing to the local docker registry needs these steps:

• sudo docker login http://mydockerregistry.foo.com:8080

  Login to the docker registry mydockerregistryserver.

• sudo docker tag eclipse_for_foo http://mydockerregistry.foo.com:8080/eclipse_for_foo eclipse_for_foo

  Tag the local image/prepare it for pushing to registry server.

• sudo docker push http://mydockerregistry.foo.com:8080/eclipse_for_foo

  Push the image to registry server.

Pulling the image from the registry server

Now that the image has been pushed to the registry server it can be pulled and run by others users using the docker pull and/or run commands:

sudo docker pull http://mydockerregistry.foo.com:8080/eclipse_for_foo

sudo docker run --name eclipse_for_foo -it -v /home/joemat/development:/share/development -e DISPLAY -v /tmp/.X11-unix:/tmp/.X11-unix http://mydockerregistry.foo.com:8080/eclipse_for_foo

Once the user has created the container, (s)he can restart the container with:

sudo docker start -i eclipse_for_foo

Mission accomplished :)

For more and detailed information see The Docker User's Guide and The Docker Book.