Add UBI base image support to Dockerfiles

[ciarams87]

Openshift certification requires that the images of certified containers are ubi based images.

We need to modify the existing Dockerfiles to support both Alpine and UBI base images using build args so that we can build UBI variants required for OpenShift certification.

Acceptance Criteria:

• The ngf, nginx, and nginx plus Dockerfiles uses ARG BUILD_OS or similar with conditional logic to build both alpine and UBI image types (see https://github.com/nginx/kubernetes-ingress/blob/main/build/Dockerfile for an example in NIC)
• Both image variants have identical functionality
• Image tags clearly distinguish Alpine vs UBI (e.g., -ubi suffix)

Notes: Use Minimal UBI based images if possible. Verification can be done on the kind cluster.