* Fix old tag of custom error pages used in example
* Move nginx-errors to k8s registry
Since the setup for the custom-error-messages was really different from
the other images that are build using cloudbuild, I changed it to "fit
in better"
* Use Go version 1.17 for custom-error-pages
Since Go >= 1.16 required the use of modules, I also initialized the module using the name k8s.io/ingress-nginx/custom-error-pages
