Docker docs:
Docker routes container traffic in the nat table, which means that packets are diverted before it reaches the INPUT and OUTPUT chains that ufw uses. Packets are routed before the firewall rules can be applied, effectively ignoring your firewall configuration.
Well yea ofc it works like that, the services are not on the same network, so the packets need to be sent onto another adapter. That means either nat or forwarding tables.
Now if that was a good design of docker is another question.