[CI] Update from pacxx-runtime.

This basically replaces the old rudimentary CI by the one from PACXX-runtime, with minimal changes.

Adresses: https://gitlab.dune-project.org/joe/pacxx-docker/issues/20, https://gitlab.dune-project.org/joe/pacxx-docker/issues/23, https://gitlab.dune-project.org/joe/pacxx-docker/issues/24