GitLab

The automatically generated hostname is too long: OpenMPI uses it as
part of a path to UNIX sockets, so there is a length limitation.

Debian stretch now has DUNE 2.5.

Currently build from the Git branch, but might switch to using packages
later.

Before the unset `DUNECI_OPTS` was used in the next line...

Otherwise no opts file is used at all and the code is built without any
optimization, leading to timeouts in tests.

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 allows using the script for `dune-common` as well.

The images are now build via `.gitlab-ci.yml` and I do not want to
continue providing the `Makefile` as well.