Improve doc for gitlab runner docker

What does this Merge Request do?

This MR specifies which port to configure when using gitlab-runner with docker on macOS

Merge Request checklist

• [-] Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.

cc @stanhu @iamricecake