A brand new from scratch installation in Docker installs fine and comes up and registers. The mongo 4.0 deprecation error persists even on a BRAND NEW installation from scratch. The resulting installation works fine for the admin, but no new users can join. All emails entered are invalid and the register a new account button never lights up.
This is not a bad thing if the admin is planned to be the only user. Everything works fine and the workspace is registered and I can install apps too. I guess a system with no users will have no complaints.