How to resolve Pups exec errors during Discourse bootstrap

OK, I fixed the separation of web_only and redis. Error msg now is

FAILED
--------------------
Pups::ExecError: cd /var/www/discourse && su discourse -c 'bundle exec rake db:migrate' failed with return #<Process::Status: pid 981 e
xit 1>
Location of failure: /usr/local/lib/ruby/gems/3.3.0/gems/pups-1.2.1/lib/pups/exec_command.rb:132:in `spawn'
exec failed with the params {"cd"=>"$home", "tag"=>"migrate", "hook"=>"db_migrate", "cmd"=>["su discourse -c 'bundle exec rake db:migra
te'"]}
bootstrap failed with exit code 1
** FAILED TO BOOTSTRAP ** please scroll up and look for earlier error messages, there may be more than one.
./discourse-doctor may help diagnose the problem.
801049b69a89d38b1ae5c299d356fc5f8dc6a8f518b1260c2dde05e0b6081556

But maybe its a misunderstanding / lack of knowledge on my side:

The database should be external on another lxc container which has a postgresql database. The database user and database exists, but the database is empty before the first bootstrap of web_only. Does the script create the database itself on the remote system upon first build? Or do I first have to create the database container and then export its default scheme and the data manually to the external postgresql daemon?

Visualization of the overall setup

forum2 Setup.excalidraw