Add alma9 images

We are going to jump directly from CentOS 7 to AlmaLinux 9 (may be calling it centos9...), so I added the configuration files to build and alma9 image.

I also removed slc5 and slc6 from the configuration as is seems they cannot be built anymore (they cannot talk to the webservers hosting packages).

Instead of trying to use the same model as the old images I decided to build only a -devel image from basic principles, also because yum has been replaced by dnf and it was too complicated to maintain a single list of packages that could (almost) work on all OSs.

I took the occasion to switch from stages to dependencies in the gitlab-ci pipeline and to invert the logic of image tagging, similar to what is done in LbDocker.