Comment 6 for bug 1755455

There appears to currently be only a single way to get the docker-distribution to listen when dealing with ipv6 traffic. You can configure it to only listen on the port which will result in it answering on any ipv4 or ipv6 interface on the system. You can specify the following hieradata in a file to accomplish this configuration:

tripleo::profile::base::docker_registry::registry_host: ''

Once you have a file with this configuration, update the undercloud.conf to reference the hieradata override file and run the undercloud installation.

Closing as won't fix as we have replaced docker-distribution with something else in newer versions which does support ipv6. The mentioned workaround should work for <=Rocky but it does open the registry on the undercloud to access from all interfaces so make sure to update firewall rules appropriately.