TL;DR
Check all nginx or servers implicated. In my case, 3 to count: website, discourse docker, and inside discourse docker. client_max_body_size should be set to the limit you want to have when uploading.
I chose ‘client_max_body_size 999m’ just because I wanted to limit it from discourse settings only. You can add that line inside the http or server tags inside nginx.conf. I prefer to add it on the last line, just to be safe 
Longer version:
After an entire day of digging and downloading firefox… totally recommend it for web dev. I found out that the reason I couldnt upload was because of the limit set by the nginx inside the docker container of discourse (some might say yeah.. but that is dynamically set when launcher is rebuilt… - i like to be sure ). Then there was the fact that our own nginx that hosts the docker image didnt have a limit set, that defaulted to ‘client_max_body_size 1MiB’, which is not good enough :). After changing that too, I had to go on the frontend nginx and change that too. 3 changes, 6 hours of dabbling with CORS pre-flight request management in nginx and a bit of stress, it now works.