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Using Docker Images

GitLab CI in conjunction with GitLab Runner can use Docker Engine to test and build any application.

Docker is an open-source project that allows you to use predefined images to run applications in independent "containers" that are run within a single Linux instance. Docker Hub has a rich database of pre-built images that can be used to test and build your applications.

Docker, when used with GitLab CI, runs each build in a separate and isolated container using the predefined image that is set up in .gitlab-ci.yml.

This makes it easier to have a simple and reproducible build environment that can also run on your workstation. The added benefit is that you can test all the commands that we will explore later from your shell, rather than having to test them on a dedicated CI server.

Register docker runner

To use GitLab Runner with docker you need to register a new runner to use the docker executor:

gitlab-ci-multi-runner register \
  --url "https://gitlab.com/" \
  --registration-token "PROJECT_REGISTRATION_TOKEN" \
  --description "docker-ruby-2.1" \
  --executor "docker" \
  --docker-image ruby:2.1 \
  --docker-postgres latest \
  --docker-mysql latest

The registered runner will use the ruby:2.1 docker image and will run two services, postgres:latest and mysql:latest, both of which will be accessible during the build process.

What is image

The image keyword is the name of the docker image that is present in the local Docker Engine (list all images with docker images) or any image that can be found at Docker Hub. For more information about images and Docker Hub please read the Docker Fundamentals documentation.

In short, with image we refer to the docker image, which will be used to create a container on which your build will run.

What is service

The services keyword defines just another docker image that is run during your build and is linked to the docker image that the image keyword defines. This allows you to access the service image during build time.

The service image can run any application, but the most common use case is to run a database container, eg. mysql. It's easier and faster to use an existing image and run it as an additional container than install mysql every time the project is built.

You can see some widely used services examples in the relevant documentation of CI services examples.

How is service linked to the build

To better understand how the container linking works, read Linking containers together.

To summarize, if you add mysql as service to your application, the image will then be used to create a container that is linked to the build container.

The service container for MySQL will be accessible under the hostname mysql. So, in order to access your database service you have to connect to the host named mysql instead of a socket or localhost.

Overwrite image and services

See How to use other images as services.

How to use other images as services

You are not limited to have only database services. You can add as many services you need to .gitlab-ci.yml or manually modify config.toml. Any image found at Docker Hub can be used as a service.

Define image and services from .gitlab-ci.yml

You can simply define an image that will be used for all jobs and a list of services that you want to use during build time.

image: ruby:2.2

services:
  - postgres:9.3

before_script:
  - bundle install

test:
  script:
  - bundle exec rake spec

It is also possible to define different images and services per job:

before_script:
  - bundle install

test:2.1:
  image: ruby:2.1
  services:
  - postgres:9.3
  script:
  - bundle exec rake spec

test:2.2:
  image: ruby:2.2
  services:
  - postgres:9.4
  script:
  - bundle exec rake spec

Define image and services in config.toml

Look for the [runners.docker] section:

[runners.docker]
  image = "ruby:2.1"
  services = ["mysql:latest", "postgres:latest"]

The image and services defined this way will be added to all builds run by that runner.

Define an image from a private Docker registry

Starting with GitLab Runner 0.6.0, you are able to define images located to private registries that could also require authentication.

All you have to do is be explicit on the image definition in .gitlab-ci.yml.

image: my.registry.tld:5000/namepace/image:tag

In the example above, GitLab Runner will look at my.registry.tld:5000 for the image namespace/image:tag.

If the repository is private you need to authenticate your GitLab Runner in the registry. Learn how to do that on GitLab Runner's documentation.

Accessing the services

Let's say that you need a Wordpress instance to test some API integration with your application.

You can then use for example the tutum/wordpress image in your .gitlab-ci.yml:

services:
- tutum/wordpress:latest

When the build is run, tutum/wordpress will be started and you will have access to it from your build container under the hostname tutum__wordpress.

The alias hostname for the service is made from the image name following these rules:

  1. Everything after : is stripped
  2. Slash (/) is replaced with double underscores (__)

Configuring services

Many services accept environment variables which allow you to easily change database names or set account names depending on the environment.

GitLab Runner 0.5.0 and up passes all YAML-defined variables to the created service containers.

For all possible configuration variables check the documentation of each image provided in their corresponding Docker hub page.

Note: All variables will be passed to all services containers. It's not designed to distinguish which variable should go where.

PostgreSQL service example

See the specific documentation for using PostgreSQL as a service.

MySQL service example

See the specific documentation for using MySQL as a service.

How Docker integration works

Below is a high level overview of the steps performed by docker during build time.

  1. Create any service container: mysql, postgresql, mongodb, redis.
  2. Create cache container to store all volumes as defined in config.toml and Dockerfile of build image (ruby:2.1 as in above example).
  3. Create build container and link any service container to build container.
  4. Start build container and send build script to the container.
  5. Run build script.
  6. Checkout code in: /builds/group-name/project-name/.
  7. Run any step defined in .gitlab-ci.yml.
  8. Check exit status of build script.
  9. Remove build container and all created service containers.

How to debug a build locally

Note: The following commands are run without root privileges. You should be able to run docker with your regular user account.

First start with creating a file named build script:

cat <<EOF > build_script
git clone https://gitlab.com/gitlab-org/gitlab-ci-multi-runner.git /builds/gitlab-org/gitlab-ci-multi-runner
cd /builds/gitlab-org/gitlab-ci-multi-runner
make
EOF

Here we use as an example the GitLab Runner repository which contains a Makefile, so running make will execute the commands defined in the Makefile. Your mileage may vary, so instead of make you could run the command which is specific to your project.

Then create some service containers:

docker run -d --name service-mysql mysql:latest
docker run -d --name service-postgres postgres:latest

This will create two service containers, named service-mysql and service-postgres which use the latest MySQL and PostgreSQL images respectively. They will both run in the background (-d).

Finally, create a build container by executing the build_script file we created earlier:

docker run --name build -i --link=service-mysql:mysql --link=service-postgres:postgres ruby:2.1 /bin/bash < build_script

The above command will create a container named build that is spawned from the ruby:2.1 image and has two services linked to it. The build_script is piped using STDIN to the bash interpreter which in turn executes the build_script in the build container.

When you finish testing and no longer need the containers, you can remove them with:

docker rm -f -v build service-mysql service-postgres

This will forcefully (-f) remove the build container, the two service containers as well as all volumes (-v) that were created with the container creation.