Improve bundler caching in CI #2224
Description
Currently our CI caches gems manually, but https://github.com/ruby/setup-ruby/?tab=readme-ov-file#caching-bundle-install-manually says
It is also possible to cache gems manually, but this is not recommended because it is verbose and very difficult to do correctly. There are many concerns which means using actions/cache is never enough for caching gems (e.g., incomplete cache key, cleaning old gems when restoring from another key, correctly hashing the lockfile if not checked in, OS versions, ABI compatibility for ruby-head, etc). So, please use bundler-cache: true instead and report any issue.
The issue is that we have multiple Gemfiles (5 currently: root, react_on_rails, react_on_rails_pro, react_on_rails/spec/dummy, react_on_rails_pro/spec/dummy), but the recommended solution is ruby/setup-ruby#170 (comment):
Multiple uses: ruby/setup-ruby should work for that purpose, it's already used by some people.
An alternative is to make a single reusable action which can be passed the path to Gemfile.lock and takes care of caching internally.