By switching to the upstream image, it's no longer needed to build an own image

It's currently not used, so why keep it around?

This patch reworks how the infrastructure setup is organised to provide
more flexibility when maintaining multiple clusters with the same
repository as well as resolving various bootstrapping issues.

The new structure will move various overwrites into the cluster-specific
infrastructure directory, mimicking the apps directory. Currently it's
quite split between infrastructure and the cluster directory.