Use the simple Ruby base image on CI rubocop job

What does this MR do?

Use the simple Ruby base image instead of one prepared for gitlab-com/www-gitlab-com on CI rubocop job.

This MR makes the CI job faster from 01:57 (before, cached) to 00:33 (after, cached).

Related issues

Closes #2639 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• [n/a] Test plan indicating conditions for success has been posted and passes
• [n/a] Documentation created/updated
• [n/a] Tests added
• [n/a] Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened