Add support for tmpfs on the job container.

What does this MR do?

For the sake of completeness, add TMPFS support for the job container. This completes @mion00 work on https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/605.

Why was this MR needed?

See #1454

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

No.

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?

#1454

Edited by username-removed-89670