Oh. That’s from your proxy nginx, not the discourse nginx. Sorry.
I don’t see anything obviously wrong in what you’ve described, but there are lots of little pieces. Maybe discourse is on the wrong docker network (but it looked like you tried you address that).
For the past several months I’ve been meaning to figure out how to do just what you’re saying, but people keep paying me to do other things. Maybe in the next week or two, but I’ve been saying that for months.