This is really ugly but it works, text is tested on registry:2.5.1. I did not manage to get delete working smoothly even after updating configuration to enable delete. The ID was really difficult to retrieve, had to login to get it, maybe some misunderstanding. Anyway, the following works:
Login to the container
docker exec -it registry sh
Define variables matching your container and container version:
export NAME="google/cadvisor"
export VERSION="v0.24.1"
Move to the the registry directory:
cd /var/lib/registry/docker/registry/v2
Delete files related to your hash:
find . | grep `ls ./repositories/$NAME/_manifests/tags/$VERSION/index/sha256`| xargs rm -rf $1
Delete manifests:
rm -rf ./repositories/$NAME/_manifests/tags/$VERSION
Logout
exit
Run the GC:
docker exec -it registry bin/registry garbage-collect /etc/docker/registry/config.yml
If all was done properly some information about deleted blobs is shown.