Add rubocop job to pipeline
There is a bit of ruby in the repository, we should use rubocop to help maintain a consistent style.
Using stock rubocop
0.69.0 we currently see:
$ rubocop ... 24 files inspected, 832 offenses detected
We can autofix some of these
$ rubocop -a ... 24 files inspected, 832 offenses detected, 493 offenses corrected
These numbers could be lowered somewhat more if we were to borrow the rubocop setup from omnibus-gitlab, or other GitLab projects. But it would still require some work to get it passing.