code: func (d *Directive) setPlatformToken(s string) error {
  	s = strings.ToLower(s)
  	valid := []string{runtime.GOOS}
  	if system.LCOWSupported() {
  		valid = append(valid, "linux")
  	}
  	for _, item := range valid {
  		if s == item {
  			d.platformToken = s
  			return nil
  		}
  	}
  	return fmt.Errorf("invalid PLATFORM '%s'. Must be one of %v", s, valid)
  }
docs: // setPlatformToken sets the default platform for pulling images in a Dockerfile.