Merge branch 'docu/add-gitlab-ci-service' into 'master'

Add documentation for using gitlab service in CI

See merge request gitlab-org/gitlab!58534

doc/ci/services/gitlab.md

+---
+stage: Verify
+group: Runner
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+type: reference
+---
+
+# Using GitLab
+
+As many applications depend on accessing JSON apis you eventually need them in order for your tests to run.
+In this example we are providing GitLab as a Microservice to be accessible for API clients.
+Below you are guided how to do this with the Docker executors of GitLab Runner.
+
+## Use GitLab with the Docker executor
+
+If you're using [GitLab Runner](../runners/README.md) with the Docker/Kubernetes executor,
+you basically have everything set up already.
+
+First, in your `.gitlab-ci.yml` add:
+
+```yaml
+services:
+  - name: gitlab/gitlab-ce:latest
+    alias: gitlab
+
+variables:
+  GITLAB_HTTPS: "false"             # ensure that plain http will work
+  GITLAB_ROOT_PASSWORD: "password"  # in order to access the api with user root:password
+```
+
+To set values for the `GITLAB_HTTPS`, `GITLAB_ROOT_PASSWORD`,
+[assign them to a variable in the user interface](../variables/README.md#project-cicd-variables),
+then assign that variable to the corresponding variable in your
+`.gitlab-ci.yml` file.
+
+From your ci `script:` the API will then be availible at `http://gitlab/api/v4`
+
+If you're wondering why we used `gitlab` for the `Host`, read more at
+[How services are linked to the job](../docker/using_docker_images.md#extended-docker-configuration-options).
+
+You can also use any other Docker image available on [Docker Hub](https://hub.docker.com/u/gitlab).
+
+The `gitlab` image can accept some environment variables. For more details,
+see the [omnibus documentation](../../install/README.md).

doc/ci/services/index.md

@@ -18,6 +18,7 @@ case is to run a database container, for example:
 - [MySQL](mysql.md)
 - [PostgreSQL](postgres.md)
 - [Redis](redis.md)
+- [GitLab](gitlab.md) as an example for a microservice offering a JSON API
 
 It's easier and faster to use an existing image and run it as an additional container
 than to install `mysql`, for example, every time the project is built.