The basic docker image does not contain any of the optional packages

Forks can disable the CI by specifying a non existing custom configuration
file e.g. '.disable-ci.yml'. This is a simple but ugly work-around.
In the GitLab sidebar one can select
Settings -> CI/CD -> General Pipelines -> Custom CI configuration path

The default shared runner does not work with the project any more, so every
fork has to set up its own runner. Pipelines will be stuck otherwise. The
project-wide CI configuration .gitlab-ci.yml defines the CI for merge requests
and the dev branch.