Investigate GitLab-CI services don't reach each other

According to the docs, services should be able to communicate with each other, if docker networks for each job are activated. But @hda11597 and me @neil.schark did not get it to work. So I implemented a workaround, so that our tests still run. But it would be better to get the services running correctly, as it would mean a better test set up.

https://docs.gitlab.com/ee/ci/services/#connecting-services

https://code.fbi.h-da.de/danet/minimal-setup-docker-gitlab-ci

https://gitlab.com/gitlab-org/gitlab-runner/-/issues/1042

Typical pipeline where it didn't work: https://code.fbi.h-da.de/danet/gosdn/-/jobs/783870

Pipeline where it ran once for whatever reason (never before and never since): https://code.fbi.h-da.de/danet/gosdn/-/jobs/783590