Okay I can’t help you further with that, sorry.
Please ensure your proxy in front of the rails container is absolutely transparent!
You might want to fiddle arround with port exposing of the railsserver (See https://github.com/zammad/zammad-docker-compose/blob/master/containers/zammad/docker-entrypoint.sh for the ports (3000 and 6042)) which might help you out of your issues.
You might also have issues with websockets through two nginxes.
Also please note that you might want to check if the FQDN-Settings (and protocol) are correct within Zammad.
Again, this is not application based, but docker based which we can’t help you with. (at least I won’t, sorry)