Fixes #2338: k8s entrypoint configuration

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What does this MR do?

Fixes #2338 (closed)

Why was this MR needed?

gitlab-runner should respect the ENTRYPOINT from the Dockerfile if no entrypoint is definied in the gitlab-ci.yml file

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

Sure. First time contribution, no golang experience.

But I test the Build on our gitlab infrastructure https://hub.docker.com/r/jkroepke/gitlab-runner and I got no problems.

Known Issues:

• Secret variables are not available if ENTRYPOINT is executed. Because they are not defined/included in the Pod Manifest for the runner pod. See https://gitlab.com/gitlab-org/gitlab-runner/blob/master/executors/kubernetes/executor_kubernetes.go#L195

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?