This tool implements a superset of the commands under cmd/crane, with
additional commands that are specific to gcr.io.
Note that this relies on some implementation details of GCR that are not consistent with the registry spec, so this may break in the future.
gcrane ls exposes a more complex form of ls than crane, which allows for listing
tags, manifests, and sub-repositories.
gcrane gc will calculate images that can be garbage-collected.
By default, it will print any images that do not have tags pointing to them.
This can be composed with gcrane delete to actually garbage collect them:
gcrane gc gcr.io/${PROJECT_ID}/repo | xargs -n1 gcrane delete