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.
Nat is not security.
Keep that in mind.
It's just a crutch ipv4 has to use because it's not as powerful as the almighty ipv6
Counterpoint