Use of docker in tests is problematic

[valeriupredoi]

@bnlawrence I would strongly suggest we don't use docker in the test suite and we don't import it as a main dependency, but rather, build a MinIO client from the command line inside a eg GHA test session like we do in PyActiveStorage via a container: https://github.com/NCAS-CMS/PyActiveStorage/actions/runs/19317279374/job/55251175054 - docker is really not liked on HPCs, and it also needs all manners of privileges that none of the CI test machines allow for (unless running a dedicated docker session), and eg not even my computer likes it much, so all the docker/MinIO tests here fail for me.

[bnlawrence]

No one is going to be developing this software on HPC, so docker in the test suite doesn't seem that problematic. That said, I don't have a problem with another technique at some time in the future.