Change Docker base images from Ubuntu to Debian, and use `snapshot.debian.org` as apt source list.
Ubuntu occasionally removes version-pinned packages from apt (see #7484), which
- breaks historical reproducible builds
- introduces maintenance burden as we have to update the version pins
Hopefully this change fixes both issues.
merges https://github.com/spesmilo/electrum/pull/7926
closes https://github.com/spesmilo/electrum/issues/7484