Add instructions for how to specify what user a job is run as via docker executor
What does this MR do?
Explains how to run a job via the docker executor with a user other than root
Why was this MR needed?
As per #2750 (comment 944572438) it's not clear to most people that a docker user can be configured or specified by the user (aka: they can run a job as a user other than root)
What's the best way to test this MR?
Read it. Try running it if you're so inclined.