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.
Yeah, I know that now lol, but good idea to spell it out. So what Docker does, which is so confusing when you first discover the behaviour, is it will bind your ports automatically to 0.0.0.0 if all you specify is 27017:27017 as you port (without an IP address prefixing). AKA what the meme is about.