I too read that it didn’t work with docker but that was not my experience on Ubuntu 24.04. Maybe it’s just docker desktop but I had all sorts of other issues with docker desktop and ditched it for plain docker, using lazydocker for an interface. I think one of the issues for outgoing connections at least is that IPs for the containers can change. I don’t remember exactly what my setup is currently but you shouldn’t have an issue opening up just specific ports mapped to a container while having default deny incoming. Not specific to containers but there are a few different Linux commands to get all ports and the processes listening on them, I used them extensively to debug my firewall setup. I can’t remember them either off the top of my head but you should be able to google for your distro.