The Docker Registry used by RegistryUI runs as the registry container (default name).
It is possible to use your already existing registry data. In your registry container, change the following volumes definition to point to the host location where the original registry data is located:
registry: container_name: registry . . . volumes: - /existingdata/registry:/var/lib/registry
This MUST match the volumes definition in the registryuiweb container, as the registry data folder must be shared between the website and the registry. The website only requires read/only access.
registryuiweb: container_name: registryuiweb . . . volumes: - /existingdata/registry:/var/lib/registry:ro
Remove your existing registry container and only allow the RegistryUI registry container to be active.
After restarting all containers (registryuiweb, registryuisql and registry), your existing registry will be accessible to RegistryUI. Make sure to synchronize the RegistryUI database with the registry, otherwise your repositories will not be shown (see Registry > Settings, Diagnostics Tab).