This is the closest absolute conversation from the original CircleCI 2.0
version as possible. Main differences:

* Actions' Postgres container requires a password or otherwise doesn't
  start (see SO answer),
* Ports are explicitly exported,
* Actions has a stronger, built-in approach to service health,
* Using the "global" environment variables stops us having to repeat the
  Postgres config multiple times,

https://docs.github.com/en/actions/guides/building-and-testing-ruby
https://docs.github.com/en/actions/guides/creating-postgresql-service-containers
https://docs.github.com/en/actions/reference/environment-variables#about-environment-variables
https://docs.github.com/en/actions/guides/creating-redis-service-containers#running-jobs-directly-on-the-runner-machine
https://stackoverflow.com/a/60618750/83386
https://www.postgresql.org/docs/9.5/libpq-envars.html