code: func (pm *Manager) GC() {
  	pm.muGC.Lock()
  	defer pm.muGC.Unlock()
  
  	whitelist := make(map[digest.Digest]struct{})
  	for _, p := range pm.config.Store.GetAll() {
  		whitelist[p.Config] = struct{}{}
  		for _, b := range p.Blobsums {
  			whitelist[b] = struct{}{}
  		}
  	}
  
  	pm.blobStore.gc(whitelist)
  }
docs: // GC cleans up unreferenced blobs. This is recommended to run in a goroutine