Naturally, I just needed to ask a question in order to stumble upon an answer. To provide information to anyone looking for this issue, I will post here a few details.
Relevant lines from nginx.conf file:
user www data; # so that nginx does not start as root
www-data of passenger_default_user; # also for passengers
root / opt / foo / app / current / public;
, , www-, config/environment.rb, , -, , , . , _default_user ? , , .
, , www- - 0700, .