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.

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

This image is used by the CI to build other docker images for the CI.

This helps understanding what happened.