Hm. Personally, I’d go without the web panel and simple run Discourse directly, but this should work too (although I don’t get which database you want to manage…).
Since your’re running a custom setup, I’d simply skip the discourse-setup script, manually copy samples/standalone.yml to containers/app.yml and edit it there. You can use the tutorial you linked to or this one which uses nginx as the front end. I prefer the latter, but that’s entirely subjective 