Add a Debian bullseye Docker build

Our target system is running Debian bullseye. Since xonstat is not fully statically linked (libcairo2, for badges), this MR adds a "build in docker" task to the Makefile to ensure we can at least compile there. The built executable is copied from inside the container to the host system when done.

This isn't fail-safe, of course. As noted in Matrix chat, the target system also has to have the right packages installed. Fortunately we're not in the business of adding or removing those too often. This'll have to do for now.