This is easier than setting CMAKE_FLAGS directly in .gitlab-ci.yml and
allows to change options in a central place (or two places).

It is used by tests in dune-fufem.

This works around https://bugs.debian.org/813000

When only leaf images are rebuilt, the Packages index might be outdated
and refer to packages that no longer exist in the archive. Always
running "apt-get update" before installing packages avoids this.

Also remove contents of /var/lib/apt/lists as we no longer use it and it
allows to keep the images slightly smaller.

These are useful to build packages that do not depend on the DUNE core
modules like UG or the core modules themselves.