Switch image building infra to GitHub Actions

[eguiraud]

This would solve #36 , but it would also let us script many of the operations that are currently performed manually when publishing new images: the repo's README file could be updated automatically and in principle we could also script github actions that check whether a ROOT version is available for download or for installation in conda/fedora/arch and, if yes, adds the tag to the repo, builds the image and publishes it.

EDIT: oh and testing: github actions could test the images by running a simple ROOT workload.

[lobis]

@eguiraud

So I posted a comment before reading this issue which is a better match. It even has the help wanted tag. You can assign me to this issue and #36 if you want.

[eguiraud]

Hey, thanks a lot for the help, I will take a look at the PR as soon as I have time