[New Container] Replace the `potentialworkshop` image by a `potentialfit` image

[mbruns91]

The potentialworkshop image is based on the potentials workshop from 2021. Shall we maybe replace it by an image providing developments from pyiron_potentialfit?

[jan-janssen]

Yes, this is the intention of the pull request #138 - still at the current stage the pyiron_potentialfit repository is not yet used in production. Furthermore, we might want to move parts of pyiron_potentialfit to the atomistics package to simplify the integration with pyiron_workflow in future.