Use GitLab CI services instead of Compose for rexec testing

Background

The following discussion from !1239 (merged) should be addressed:

I was initially thinking of spinning up the grid using GitLab CI services but two limitations prevent us from doing that:

• The worker container need privileges and this is not supported in .gitlab-ci.yml at the moment:
  • Upsteam issue: gitlab-org/gitlab-runner#238 (closed).
• The worker container need network access to the controller container and this can only be achieve using a workaround at the moment because GitLab CI uses --link for services:
  • Upsteam issue: gitlab-org/gitlab-runner#1042 (closed).
  • WiP merge request: gitlab-org/gitlab-runner!1041 (closed).
• Logs for GitLab CI services are not currently available:
  • Upsteam issue: gitlab-org/gitlab-runner#2119 (closed).

I believe this could be made a lot more neat once these issues are resolved upstream.

Task description

Once GitLab CI services are mature enough:

• Setup BuildGrid as a GitLab CI service.
• Drop the Docker Compose setup.

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Related to #629 (closed) and !1239 (merged).