By using `podman` and eliminating the need for a docker
image registry proxy wiith the new core ci setup, it's
no longer required to push image builds to DockerHUB.

Due to changes to the docker hub policies for open source
projects a couple of month ago, we are finally able to
drop the publishing of images to the Docker HUB registry.

This change removes the build and push towards the docker
HUB registry in the `Makefile` and adjusts the documentation
in the `README.md`