The Isolated Cluster API was implemented with using the deprecated registry.mirrors configuration.

NOTE: registry.mirrors and registry.configs as previously described in this document have been DEPRECATED. As described in the cri config you should now use the following configuration

https://github.com/containerd/containerd/blob/main/docs/cri/registry.md

Working example with the new configuration: https://github.com/robertvolkmann/containerd-registry-mirror