code: func (db *distBundle) setAppDefaults() error {
  	app := db.dist.(*dist.Appc).App()
  	if _, ok := app.Labels["arch"]; !ok {
  		app.Labels["arch"] = common.GetArch()
  	}
  	if _, ok := app.Labels["os"]; !ok {
  		app.Labels["os"] = common.GetOS()
  	}
  	if err := types.IsValidOSArch(app.Labels, stage0.ValidOSArch); err != nil {
  		return errwrap.Wrap(fmt.Errorf("invalid Appc distribution %q", db.image), err)
  	}
  	db.dist = dist.NewAppcFromApp(app)
  	return nil
  }
docs: // TODO(sgotti) I'm not sure setting default os and arch also for image
  // dependencies is correct since it may break noarch dependencies. Reference:
  // https://github.com/rkt/rkt/pull/2317