Allright, thanks a lot for the detailed answer 
No way I’ll run the app as sudo, or as a user that can sudo without password, don’t worry. But it’s true that until now, on my machine, I was just using my own user to run bundle install, rails dev server, etc, for rails apps.
For Heroku: it’s easy to setup an elasticsearch service with the free Bonsai addon The free PosgreSQL database goes up to 10 million rows. I understand your point of view, but I’m not a sysadmin (and I don’t want to be one) so having a system where you just push a rails app with git push heroku master and then everything is done for you automatically (building, deploying, etc) is convenient. I’d like to try to improve/hack Zammad, so that’s why I’m searching a solution for easy deployment.
I’ll try Zammad on Heroku when I’ll have more time (perhaps next week) and if it works, I’ll publish here the instructions.