Remove `registry` from `repository` if included

[shrink]

Sometimes a user might not have the cleanest of input, i.e: they may have a full repository path (ghcr.io/shrink/example-image) and so when they provide registry or repository this action should identify that case and sanitise the input, so that user sdo not need to implement their own separate input validation.

[shrink]

Alternatively, add an additional image input which takes a full image name and extracts the registry + repository values. That's probably a saner approach, less surprising.