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Testing against the last Ubuntu LTS release might be a good idea.

It is based on Debian 8 with a recent version of Clang from the
backports repository. This allows testing recent Clang versions while
avoiding the incompatibilities between Clang and newer libstdc++
versions.

Reference: https://gitlab.dune-project.org/core/dune-common/issues/37#note_19885
Reference: https://gitlab.dune-project.org/staging/dune-functions/merge_requests/29#note_20177

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

clang-3.6 has problems with GCC-6's libstdc++, see for example the
failing build of dune-grid[1].  With clang-3.8 these problems seem to
have been solved.

  [1] https://gitlab.dune-project.org/core/dune-grid/builds/1200

It is required by the current version of `dune-solvers`.

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

Reference: https://gitlab.dune-project.org/core/dune-common/merge_requests/111#note_18188

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).