Add an image for Rocky 9

Add an image of Rocky 9. There's currently no image available in ci-tools/ci-worker> so we are taking it straight away from the Docker repositories. It is now possible to choose in the CI to build images with the latest version of TeXLive. The chance is taken to fix the broken build environment, updating TeXLive versions of the slc6 and cc7 images to the latest ones available, together with another image for cc7 with TeXLive 2020 (for backwards compatibility).

⚠ Despite we are including a Rocky 9 image, the CERN packages are not available and the environment is different from what can be found on lxplus