Build static versions of Ruby for m0ar cachingz

At the moment we use `ONBUILD` magic to read `.ruby-version` from an application and automatically compile the correct version of Ruby when the application image is built.

This strategy works well when the Docker host building the application already has a cached copy of the Ruby compilation step, but otherwise (e.g. when quay.io automatically builds `project-nylon`) it's slow as asses.

This PR opts to use Docker tags to provide fixed versions of Ruby instead. This will be much faster for cache misses. The downside is that applications need to specify a Ruby version (by way of `BASE` tag) in each application's `Dockerfile`, but given that Ruby versions rarely change, this should be a big net win.