ADD to wiki: How to host a container in a custom registry

[PhMueller]

We should add a short description to the wiki about how to link a benchmark to a custom registry.

I think we can already do that by adding something like that to the benchmark's container interface:
(e.g.: https://github.com/automl/HPOBench/blob/master/hpobench/container/benchmarks/nas/nasbench_1shot1.py)

kwargs['container_source'] = kwargs.get('container_source', <URL_CUSTOM_REGISTRY>)

e.g.:
kwargs['container_source'] = kwargs.get('container_source', 'oras://gitlab.tf.uni-freiburg.de:5050/muelleph/hpobench-registry')