code: func (w *Worker) Exec(ctx context.Context, meta executor.Meta, rootFS cache.ImmutableRef, stdin io.ReadCloser, stdout, stderr io.WriteCloser) error {
  	active, err := w.CacheManager.New(ctx, rootFS)
  	if err != nil {
  		return err
  	}
  	defer active.Release(context.TODO())
  	return w.Executor.Exec(ctx, meta, active, nil, stdin, stdout, stderr)
  }
docs: // Exec executes a process directly on a worker