code: func maintainer(b *Builder, args []string, attributes map[string]bool, flagArgs []string, original string) error {
  	if len(args) != 1 {
  		return errExactlyOneArgument("MAINTAINER")
  	}
  	b.Author = args[0]
  	return nil
  }
docs: // MAINTAINER some text <maybe@an.email.address>
  //
  // Sets the maintainer metadata.