Switch GESIS node to also use registry.2i2c.mybinder.org

[yuvipanda]

I just realized the gesis node uses dockerhub, rather than its own registry for images. Given it's already pushing out of cluster, we should have it use registry.2i2c.mybinder.org instead. This means fewer rebuilds, and less complexity overall. And I don't expect pull performance to be fundamentally different, although we may have to keep an eye on network transfer fees. Thankfully hetzner is much better than the major cloud providers about that so we may be ok.

[rgaiacs]

Because the mybinder.org GESIS node runs on GESIS premises, one complain that we receive from GESIS IT is that we are responsible for a huge network traffic. This is due to container images being pushed and pulled. @arnim and I are investigating the options to run a local container registry, see #3087 and jupyterhub/binderhub#1904.

@arnim what do you think?

[arnim]

We first need to decide whether the Docker images are solely an intermediate cash or if they play an important role in reproducibility for us. This decision will probably be made within one month.

[manics]

This is the overlap in number of repositories between federation members for this year so far:
https://gist.github.com/manics/dc8b7547e0d60e6d215bb41ad60f9988