Insufficient privileges for Passenger spawned application

In debian, passenger runs as the owner of /usr/share/redmine/config/environment.rb,
hence this snippet in postinst :
chown www-data:root /usr/share/redmine/config/environment.rb

Isn't that the case of passenger in ubuntu ?

Passenger runs as root on Ubuntu (which is the owner of /usr/share/redmine/config/environment.rb). The snippet you include does not seem to have run because environment.rb has ownership root.root. Either way, Passenger spawns the ruby application as user defined in /etc/apache2/mods-available/passenger.conf.

I'd be surprised if passenger would allow itself to run as root. Anecdotal evidence suggests it isn't as on my setup, before I added the PassengerDefaultUser record to passenger.conf my setup was complaining that it didn't have write access to /etc/redmine/default/session.yml, which was owned by root. I changed it to be owned by www-data and it still complained, but adding the PassengerDefaultUser setting fixed it.

(I'd guess that it passenger becomes nobody if it finds that it is root).
So either way a fix is needed, either:-
1) Add PassengerDefaultUser www-data to passenger.conf
or
2) Add code to the package script, to set the ownership of environment.rb to www-data

#1 would seem to me to be the best solution as #2 is a little obscure if you're not familiar with passenger and how it works.