code: func user(b *Builder, args []string, attributes map[string]bool, flagArgs []string, original string) error {
  	if len(args) != 1 {
  		return errExactlyOneArgument("USER")
  	}
  
  	b.RunConfig.User = args[0]
  	return nil
  }
docs: // USER foo
  //
  // Set the user to 'foo' for future commands and when running the
  // ENTRYPOINT/CMD at container run time.
  //