Add userns support for Docker executor

What does this MR do?

Fixes #2314 (closed) by adding support for userns mode to the Docker executor.

Why was this MR needed?

See issue.

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

I have never coded in Go, so I do not even know if my changes are correct.

And it seems I have a bug /hence the WIP:) which I hope someone could help resolve. See the pipeline job for the Docker Executor.

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?

The issue #2314 (closed) should be fixed by this MR.