See https://gitlab.dune-project.org/joe/pacxx-docker/issues/30

Copied from pacxx-runtime!38

See: https://gitlab.dune-project.org/joe/pacxx-docker/issues/27

Copied from
pacxx-llvm2!8

See:
- https://gitlab.dune-project.org/joe/pacxx-docker/merge_requests/65
- https://gitlab.dune-project.org/joe/pacxx-docker/issues/21

Put most stuff into ci-main.yml, and include that from .gitlab-ci.yml.  This
means I can automatically verify the common stuff is up-to-date.

Adresses: https://gitlab.dune-project.org/joe/pacxx-docker/issues/25

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