* 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
Rather than hard-coding the default response format as HTML, allow the default to be overridden by an environment variable. For example, given a REST API endpoint that defaults to responding in JSON, you may wish to configure the error messages to be JSON by default as well.
This should fix issue [4039](https://github.com/kubernetes/ingress-nginx/issues/4039). This default backend fails to send the correct `Content-Type` header when it fails to decode the `Accept` request header.
This patch simply forces `text/html` in that specific scenario.
