GitLab

Reference: https://gitlab.dune-project.org/infrastructure/issues0/issues/44

For testing dune-corepy.

Based on current Debian testing and configured to use modern
compilers in C++17 mode.

This matches the name now used in `dune-common`.

We need to provide a `duneci-ctest` symlink for compatability with older
`.gitlab-ci.yml` files.

This should allow to decrease duplication in most `.gitlab-ci.yml` files.

It is used by a test in `dune-grid`.

These are needed for optional tests in dune-istl.

The `parametertreelocaletest` test requires a locale that uses a comma
instead of a dot as the decimal seperator.

Bug: https://github.com/aburch/dune-docker/issues/1

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When only leaf images are rebuilt, the Packages index might be outdated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