Software installation workflow for Habrok / CVMFS

Created by: bedroge

This adds a new eb_install.sh that uses a container with CVMFS, and builds software to a writable overlay created with fuse-overlayfs. It also contains new EB files for Habrok (EB configuration, hooks), the build container recipe, a Github Action to build the container (and store it in the Github registry), a first draft for a Habrok Easystack file (note: this has to be converted to a new structure when EB 3.7 is released), and some initial documentation. The latter still needs to be extended with instructions for setting up and running a bot, but maybe this fits better on the wiki anyway (?).