Allow User to be specified for a CI Image or Service

What does this MR do and why?

Adds User option to the newly added Docker executor options

Related: gitlab-runner!4483 (merged)

Closes gitlab-runner#2750 (closed) and #23046 (closed)

Screenshots or screen recordings

Here is an example .gitlab-ci.yml file

job1:
  image: 
    name: ubuntu
    docker:
      user: ubuntu

job2:
  image: alpine
  services:
    - name: ubuntu
      docker:
        user: ubuntu

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

/cc @furkanayhan @avonbertoldi