Docker inside docker and overview about Jenkins 2

#docker inside docker and an overview about Jenkins 2 https://t.co/qa5ddjfhrs @docker @jenkinsci #container

— Gianluca Arbezzano (@GianArb) May 4, 2016

Jenkins is one of the most famous continuous integration and deployment tools, it’s written in Java and it helps you to manage your pipeline and all tasks that help you to put your code in production or manage your build.

The announcement of Jenkins release of version 2 few days ago, is one of the best release of this year in my opinion.

The previous version is very stable but it has a lot of years and the ecosystem is totally different. I am happy to see a strong refurbishment to get the best of this powerful tool with a series of new feature like:

• Nice installation wizard
• Refactoring of the design, one of the most critical feature of the previous version
• Good and modern set of plugins like Jenkins Pipeline to manage your build

Jenkins is truly a wonder but the tool of the moment it’s docker, engine that allow you to work easier with the containers.

This two tools together are perfect to create an isolated environment to test and deploy your applications.

The first setup could be install Jenkins on your server and use a plugin to manage the integration and trigger your test inside an isolated environment, the container.

Great work but in my opinion reproducibility is one of the critical point when you deal with plugins if you can not run your build on your local environment easily then you have a problem. Secondly if the container could be a good solution to deploy and maintain a solid and isolated application, why your Jenkins has not the privilege to run inside a container? In this perspective how can we run container inside a container?

Ok, now its the time to figure it out how to solve the problems.

We can use the official Jenkins image to put jenkins inside a container, but I worked on my personal alpine installation, light and easy, here is the dockerfile and we can pull it:

docker pull gianarb/jenkins:2.0

If you are interested the main article to understand how run docker inside docker is written by jpetazzo, the idea is run our jenkins container with -privileged enabled and share our docker binary and the socket /var/run/docker.sock to manage our communications.

• /var/run/docker.sock is the entrypoint of the docker daemon
• docker the command is like a client that sends commands to socket
• --privileged give extended privileges to our container

Translated in a docker command:

docker run -v /var/run/docker.sock:/var/run/docker.sock \
    -v $(which docker):/usr/local/bin/docker \
    -p 5000:5000 -p 8080:8080 \
    -v /data/jenkins:/var/jenkins \
    --privileged \
    --restart always \
    gianarb/jenkins:2.0

We connect on http://docker-ip:8080 and start the new awesome wizard!

To verify that all work we can create a new job it only runs docker ps -a our expectation is the same list of containers that we have out of jenkins.

Now we can use run command from jenkins to manage our build with docker without any kind of plugins but anyway you are free to use Docker Plugin to start your build.

I used Jenkins like an example to run docker inside another container but you can use the same strategy to do the same with your applications if they require a strong connection with docker.