code: func (d *Directive) setEscapeToken(s string) error {
  	if s != "`" && s != "\\" {
  		return fmt.Errorf("invalid ESCAPE '%s'. Must be ` or \\", s)
  	}
  	d.escapeToken = rune(s[0])
  	d.lineContinuationRegex = regexp.MustCompile(`\` + s + `[ \t]*$`)
  	return nil
  }
docs: // setEscapeToken sets the default token for escaping characters in a Dockerfile.