[CI] Add Container Image/Workflow for Premerge Advisor

This adds a Dockerfile for building a container image for the premerge advisor
and a Github workflow similar to the other ones to build the container and
push it to GHCR.

Reviewers: cmtice

Reviewed By: cmtice

Pull Request: https://github.com/llvm/llvm-zorg/pull/606
2 files changed