code: func (wfe *WebFrontEndImpl) Issuer(ctx context.Context, logEvent *web.RequestEvent, response http.ResponseWriter, request *http.Request) {
  	// TODO Content negotiation
  	response.Header().Set("Content-Type", "application/pkix-cert")
  	response.WriteHeader(http.StatusOK)
  	if _, err := response.Write(wfe.IssuerCert); err != nil {
  		wfe.log.Warningf("Could not write response: %s", err)
  	}
  }
docs: // Issuer obtains the issuer certificate used by this instance of Boulder.