This same one got me after years of using Docker, I only discovered it after using the combination of Ubuntu Server (and it's ufw) on a DMZed device.

I was running what I thought was an internal FTP instance for almost a week. Luckily it was about as hardened as regular ftp can be, but I noticed the problem when my service wasn't able to log in as the (very low) connection limit was filled by someone attempting passwords.

I've been using https://github.com/shinebayar-g/ufw-docker-automated to make docker compliant with ufw, and defining firewall rules as labels for the containers. It needs some work still, namely the service should be hosted in it's own container for easier updating, but it works reasonably well.