Without "-v", docker would keep around volumes used by the container
forever and eventually fill the disk.

Docker keeps containers and images around after the container is no
longer in use. These eat up all disk space and need to be cleaned up
from time to time.

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When only leaf images are rebuilt, the Packages index might be outdated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.

This allows `dune-contact` to be built using this image.

It's contraproductive to cache "git pull" from the master branch as we would
not pick up changes this way.  The same applies for "apt-get update".

Some of them fail and are not yet fixed upstream...

This is the current state of the Git master branch of the DUNE core modules.
The image needs to be rebuilt from time to time.

We use a staged build to make sure to only update the image if the tests for
the core modules pass.