code: func stopSignal(b *Builder, args []string, attributes map[string]bool, flagArgs []string, original string) error {
  	if len(args) != 1 {
  		return errExactlyOneArgument("STOPSIGNAL")
  	}
  
  	sig := args[0]
  	if err := signal.CheckSignal(sig); err != nil {
  		return err
  	}
  
  	b.RunConfig.StopSignal = sig
  	return nil
  }
docs: // STOPSIGNAL signal
  //
  // Set the signal that will be used to kill the container.