blubber: add wmf-certificates deb pkg and work around image build failures
We need the wmf-certificates package to validate database connections. Additionally, as I started this work, I realized the latest version of the 'python-bookworm' image is broken. So I changed to the 'python-build-bookworm' image and started using a specific image tag instead of implicit 'latest.' This change should help image stability.
Bug: T363001