Hitting docker hub rate limits using the k8s test network

I worked around the problem by using an alternative container registry but that involved editing various scripts and config files. The fabric images were easier because `TEST_NETWORK_FABRIC_CONTAINER_REGISTRY` already provides a way to override the default container registry. I'm thinking of creating a PR to switch the default fabric container registry to `ghcr.io`, and to add a new `TEST_NETWORK_3RD_PARTY_CONTAINER_REGISTRY` env var to make it easier to use other registries.

Does that sound ok?

Docker hub is not the only container registry with rate limits though, so it would also be useful to update the sample to work with API keys, or maybe that's just a readme update?