HA!
I stand corrected: proxy_set_header X-Real-IP $remote_addr; is correct!
My problem was that i only refreshed my discourse tab instead of closing and opening a new one, and the connection was somehow kept by outer nginx.
After restarting nginx service: systemctl restart nginx, i can see the correct ips being passed to discourse.
@mpalmer can you confirm that what i did is ok, and if it is should i edit the guide/wiki and add this new header?