code: func KubernetesVersionToImageTag(version string) string {
  	allowed := regexp.MustCompile(`[^-a-zA-Z0-9_\.]`)
  	return allowed.ReplaceAllString(version, "_")
  }
docs: // KubernetesVersionToImageTag is helper function that replaces all
  // non-allowed symbols in tag strings with underscores.
  // Image tag can only contain lowercase and uppercase letters, digits,
  // underscores, periods and dashes.
  // Current usage is for CI images where all of symbols except '+' are valid,
  // but function is for generic usage where input can't be always pre-validated.