https://pagure.io/centos-infra/issue/1546

The current role for www.centos.org (and www.stg.centos.org) was just using same container to build/render output and then pushes content to real (externally facing) www hosts. (or single www.stg.centos.org host, to preview `staging` branch)

As it seems there is a need to use a different container, but also ensuring transition will be done smoothly (`staging` and `main` branches needing different containers and why to invoke these), we can modify the role to just use a container.specs file, itself in the git repo, to know which container to use and where to pull it from (as there are many registries these days)

At the same time, we'll also provision a new el9 based www.stg node (as current instance is still on el8 so good to migrate at the same time)