Speed up CI pipeline
Could create a docker image in a first stage with the conda environment, which is only rebuilt if the dependencies change.
Building the docker image may need Docker-in-Docker support, so probably a good idea to do a first simple test if we can even build a docker image in CI.
See also: https://iffgit.fz-juelich.de/docker-images/gr-build-images/-/blob/master/.gitlab-ci.yml