code: func NewCacheKey(dgst digest.Digest, output Index) *CacheKey {
  	return &CacheKey{
  		ID:     rootKey(dgst, output).String(),
  		digest: dgst,
  		output: output,
  		ids:    map[*cacheManager]string{},
  	}
  }
docs: // NewCacheKey creates a new cache key for a specific output index