Reproducibilty considerations and tweaks

[hannesm]

from #164, we suffer a bit that

• the ubuntu docker container is not reproducible per-se
• we use apt update leading to a random set of installed packages

To solve these issue, thanks to @h01ger we have some improvements (that should be done to (a) build-with-docker.sh and (b) builds.robur.coop):

• use a reproducible (debian) container from https://hub.docker.com/_/debian
• use a package snapshot (e.g. http://snapshot.notset.fr/archive/debian/20220523T033231Z/ instead of https://deb.debian.org/ for your apt sources.list) NB: snapshot.notset.fr will soon (tm) also be available as under snapshot.reproducible-builds.org but we are not there yet

[hannesm]

We could as well use orb in the GitHub action runner and collect/upload the build information.

[palainp]

Done with #175.