Allowed CI for outside Forks

See #219 for a demonstration

At the moment, it is impossible for outside forks to contribute, since part of the CI builds a container and then tries to publish this container to our ghcr.io page. 

This all boils down to badly down HDF5/NetCDF and thread-safety which is why I needed to build the CI as stupidly as I did. I am still not sure if this was the right move or not...