Allow to specify user in docker executor

Closed username-removed-1238069 requested to merge sathieu/gitlab-ci-multi-runner:docker_user into master

What does this MR do?

Add an "user" configuration to runners.docker.

Why was this MR needed?

We want to limit the permissions inside the container.

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

  • Documentation created/updated
  • Tests
    • Added for this feature/bug
    • All builds are passing
  • Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?