Build UBI images for agentk

Problem to solve

In #155 (closed) we are moving KAS builds to CNG. But agentk is not moving yet as ARM support is high priority (#155 (comment 830016829)). However, UBI images are also important for enterprise users, so we should build those as well.

Proposal

1. Tag a -ubi suffixed version of agentk images using ubi-minimal instead of distroless. See the KAS Dockerfile for a rough idea of the expected image layout.
2. Get the UBI images "certified". I am not entirely sure what this entails right now. Can also be done in a follow up.

I'm assigning this the weight 5-large because the certification step is entirely unknown. If we scoped this down to just building the image, then I would assign it a 2-small.