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. * https://gitlab.com/gitlab-org/gitlab/-/issues/415160#note_1428375072 * https://gitlab.com/gitlab-org/gitlab/-/issues/415160#note_1428375524 ## Goal Investigate and experiment to see whether we could make those jobs faster. ## Ideas ### `rspec:coverage` * [Do we need a **git clone**?](https://gitlab.com/gitlab-org/gitlab/-/issues/415160#note_1428582192) * [Do we need to install gems, or do we just need `simplecov`?](https://gitlab.com/gitlab-org/gitlab/-/issues/415160#note_1428583360) * [Could we just use an **alpine ruby image**?](https://gitlab.com/gitlab-org/gitlab/-/issues/415160#note_1428583360) ### `rspec:undercoverage` * [Could we just use an alpine ruby image?](https://gitlab.com/gitlab-org/gitlab/-/issues/415160#note_1428594580) * [Could we just install the gems we need? (e.g. `undercover` and dependencies from `Gemfile.lock`)](https://gitlab.com/gitlab-org/gitlab/-/issues/415160#note_1428595113)
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