Add support for Docker '--cpus' option

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What does this MR do?

Adds support for --cpus option for Docker executor. Successor of !469 (closed).

Why was this MR needed?

Please read #2101 (closed) for a reference

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

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?

Closes #2101 (closed)