builder image: use sane umask

What does this MR do?

Revert umask 0000 to umask 0002

• https://gitlab.com/widerin/gitlab-ci-multi-runner/commit/b712e24954de7db7150070823aad2c2ac9abb105
• https://gitlab.com/widerin/gitlab-ci-multi-runner/commit/a3484ad0450a69fd3b966f05af95e98aad6dc5a2

Why was this MR needed?

• https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1736#note_35314083
• https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/440#note_21342043

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: #1736