As this still has some relics from the age of the Node+Jekyll setup,
this needs to be cleaned up somewhat.

- The first step is to re-use the upstream `ruby` image
- It also appears that other dependencies are no longer required for
  FFI/Nokogiri, so that saves some install time
- We can remove the timezone setup as it's not needed for testing
  purposes
- We can just have `/app` as the directory, not `/app/site` as that was
  required for the previous setup.
- We can also avoid running an `apk upgrade` to save some time, for the
  tradeoff of slightly out-of-date packages