The app is handed a file handle from NGINX, so somehow NGINX is timing out cause there is some sort of timeout missing that needs to be fiddled with or something else.
What you need to do is enter your container, fiddle with nginx configuration till it works using sv nginx restart each time you fiddle with it.
But, you are creating a very very fragile solution that in the real world ™ will fail, so I don’t see why we should invest much more time in this.