[WIP] Bump Docker image to Bullseye
Since we need OpenJDK 15+ in Briar Desktop, this MR bumps the base Debian image from buster to bullseye.
As discussed in briar-desktop!44 (comment 57092).
- check that it works in briar-android
- check that it works in briar-desktop
- check that it works in briar-mailbox