code: func (w *Worker) Exporter(name string, sm *session.Manager) (exporter.Exporter, error) {
  	switch name {
  	case "moby":
  		return w.Opt.Exporter, nil
  	case client.ExporterLocal:
  		return localexporter.New(localexporter.Opt{
  			SessionManager: sm,
  		})
  	case client.ExporterTar:
  		return tarexporter.New(tarexporter.Opt{
  			SessionManager: sm,
  		})
  	default:
  		return nil, errors.Errorf("exporter %q could not be found", name)
  	}
  }
docs: // Exporter returns exporter by name