code: func NewLazySource(root containerfs.ContainerFS) (builder.Source, error) {
  	return &lazySource{
  		root: root,
  		sums: make(map[string]string),
  	}, nil
  }
docs: // NewLazySource creates a new LazyContext. LazyContext defines a hashed build
  // context based on a root directory. Individual files are hashed first time
  // they are asked. It is not safe to call methods of LazyContext concurrently.