Optimize the rspec:coverage and rspec:undercoverage jobs for speed

Context

• rspec:coverage and rspec:undercoverage are the last jobs we run on the critical path of code pipelines. As soon as they finish, the pipeline is finished.
• gitlab-org/gitlab#415160 (comment 1428375072)
• gitlab-org/gitlab#415160 (comment 1428375524)

Goal

Investigate and experiment to see whether we could make those jobs faster.

Ideas

rspec:coverage

• Do we need a git clone?
• Do we need to install gems, or do we just need simplecov?
• Could we just use an alpine ruby image?

rspec:undercoverage

• Could we just use an alpine ruby image?
• Could we just install the gems we need? (e.g. undercover and dependencies from Gemfile.lock)