Docker server's garbage collect does not delete old revisions of a tag that are inaccesible. This script will clean up the stranded revisions left behind in a docker registry to allow docker server's garbage collector to perform its task thouroughly

clean-docker-server.sh

#! /bin/bash

HELP=true
SOURCE=""
REPOSITORY="http://localhost:5000"
SOURCE_FOUND=false

while [[ $# -gt 0 ]]
do
key="$1"

case $key in
    -r)
    REPOSITORY="$2"
    shift
    ;;
    -h|--help)
      HELP=true
    ;;
    *)
      if $SOURCE_FOUND; then
        HELP=true
      else
        HELP=false
      fi
      SOURCE="$1"
      SOURCE_FOUND=true
    ;;
esac
shift # past argument or value
done

if $HELP; then
  echo "Usage: $0 [-r url] SOURCE

SOURCE      location of the repositories directory eg:
            /mnt/docker/registry/v2/repositories
-r url      url to connect to the docker repository
            defaults to: http://localhost:5000
"
  exit 1
fi

echo "Working on directory: $SOURCE and Docker Server: $REPOSITORY"

for repo in $(curl -s $REPOSITORY/v2/_catalog | jq -r '.repositories[]'); do
  echo "Repo: $repo"
  for tag in $(curl -s $REPOSITORY/v2/$repo/tags/list | jq -r '.tags[]'); do
    echo "  tag: $tag"
    current=$(cat $SOURCE/$repo/_manifests/tags/$tag/current/link | sed 's/sha256://')
    count=0
    for sha in $(ls -1 $SOURCE/$repo/_manifests/tags/$tag/index/sha256); do
      if [ "$current" = "$sha" ]; then
        echo "    found current"
      else
        count=`expr $count + 1`
        UNWANTED_REVISION=$SOURCE/$repo/_manifests/revisions/sha256/$sha/link
        UNWANTED_TAG=$SOURCE/$repo/_manifests/tags/$tag/index/sha256/$sha
        if [ -f $UNWANTED_REVISION ]; then
          rm -rf $UNWANTED_REVISION
        fi
        if [ -d $UNWANTED_TAG ]; then
          rm -rf $UNWANTED_TAG
        fi
      fi
    done
    echo "    removed $count stranded revisions"
  done
done

echo "run garbage collection to clean up the docker server
eg: registry garbage-collect /etc/docker/registry/config.yml
"