Build with sbuild

Aiming to solve some important issues, e.g.: #195 (closed) or #187 (closed), we are considering using sbuild to build the packages (now that the containers can run in privileged mode). The purpose of this issue is to document the changes and get feedback from different projects extending the pipeline (ping @mariadb-team, @kernel-team, @cpascual-guest (for ALBA), @hertzog (for kali)), to avoid breaking things (without warning :-)).

We haven't implemented any change yet. But moving to sbuild would mean, at least:

• Getting rid of the extract-source job
• Use a specific build image instead of the base
• Something else?

Note: @billy is working on this.