Support passing User to Docker Executor

Muhammed Alirequested to merge
gitlab-community/gitlab-org/gitlab-runner:user-executor-option into main

What does this MR do?

Support passing User to Docker Executor

Allow the Docker User to be specified for Build Containers and Services

Why was this MR needed?

What's the best way to test this MR?

Running whoami inside a container

What are the relevant issue numbers?

#2750 (closed)

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