I know this thread is old as hell, but i was having the same problem and had to do some tweaks to make it work.
The thing is that i checked with ./launcher logs app and turns out the onion template copies a bunch of stuff from the default site, the problem is that some of these settings are duplicated.
I had to log into the container with ./launcher enter app and then edit the /etc/nginx/conf.d/onion.conf to delete the line proxy_buffer_size 8k;
Additionally, I had to add the server_names_hash_bucket_size 64; to the file nginx.conf
This made the server work again, I found this by looking at the logs