I don’t think that word means what you think in means. Or rather, “I don’t know how to change them” does not mean “they can’t be changed”.
Oh hey, there already is a user called discourse. Which user specifically do you suggest that environment variable change? Choose carefully because they all control important data.
There are six daemons running in the container – cron, nginx, postgres, redis, syslog, and unicorn. Each of them is started by a runit/runsvdir/runsv script in /etc/service.
The “official” method to modify a container is to add run and replace stanzas in the app.yml and rebuild. (This configuration system is called “pups” if you want to search the forum here for more details.)
For example, you’d change UID/GID for the redis server by modifying the last line in the script:
exec thpoff chpst -u redis -U redis /usr/bin/redis-server /etc/redis/redis.conf