NGINX: Implement CI for UBI container generation

Summary

As part of #3374 (closed), we need to implement the UBI based container generation as a supplemental CI item within our fork of ingress-nginx, located at https://gitlab.com/gitlab-org/cloud-native/charts/gitlab-ingress-nginx

Acceptance

1. .gitlab-ci.yml is added to our fork, applied to default branch, and those which we will consume (must align)
2. Image path is clearly defined
   • Consider separate path for unofficial releases such as /staging/ to separate artifacts (context).
3. Image tag is clearly defined
   • Keep in mind that the NGINX template currently tests that the version is >=0.27.0-0 (source). This may require modifying the check depending on the format of the tag used. (Fixed in !2668 (merged)).
4. The produced image is placed into a container repository.
   • This could be within the same project, but may end up in another.
5. [ ] The BASE_IMAGE value in the Makefile is overridden in CI with the base image we build (context).
   • Moved to gitlab-org/cloud-native/charts/gitlab-ingress-nginx#1 (closed)