Adding CI workflow for building docker images and pushing to dockerhub and/or GitHub registry

[dingp]

This is not urgent. It would be good to add GitHub Action CI workflow to automate the building and publishing of docker images defined by Dockerfiles in this repo.

An example can be found at: https://github.com/DUNE-DAQ/daq-docker/blob/develop/.github/workflows/sl7-minimal.yaml
I can discuss more details or provide any help I could if needed.

Also we have a dunedaq organization with dockerhub. It would be good to have the images added to the organization.

[jcpunk]

This was resolved a ways back and can be closed out.