Bring over the rails gem cleanup from omnibus

Closes gitlab-org/distribution/team-tasks#520

See merge request gitlab-org/build/CNG!316

Ensure mailroom is updated when it's dockerfile changes

Closes gitlab-com/gl-infra/delivery#562

See merge request gitlab-org/build/CNG!319

Fixes an issue where mailroom wasn't taking into account the version of
it's project folder when determined whether a new build was needed.

This directory contains a unit test key that causing some scanning programs
to flag it

Remove unnecessary test data

So we don't need to prune here