ci(earthly): Add PoC for earthly

This patch provides some first implementations for earthly, a build system that combines Makefile and Dockerfile into a neat package(?). As a first test, this re-implements the container image build pipeline in a backwards compatible fashion and provides a first rundown in CI as well.

Currently this is build completely parallel so no changes to the existing CI system take place. In the long term it seems to have to potential to replace the current Makefile + Dockerfile jungle.