[Guide wishlist] How to make a rails app pipeline *really* fast

I'd love to see a guide that taught folks everything that goes into making a fast buildkite pipeline.

That could either be per-technology guide (like rails) or a general performance guide… or both. 🌮

A few ideas to get this moving along:

* Should we use Docker volumes?
* Add Gemfile, then `RUN bundle install`? Why?
* Docker caching between steps
* How best to parallelise the suite
* How big does a machine instance need to be to see a real performance gain?