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.
It’s my understanding that docker uses a lot of fuckery and hackery to do what they do. And IME they don’t seem to care if it breaks things.
I don’t know how much hackery and fuckery there is with docker specifically. The majority of what docker does was already present in the Linux kernel namespaces, cgroups etc. Docker just made it easier to build and ship the isolated environments between systems.