Out of the box, security with Docker (and Docker Swarm) over the network is bad. Okay, that’s not entirely true. Out of the box when you have no containers started, it’s fine. But after you start a container, and if you publish a port, they are exposed to the outside world by default. And it’s not easy to fix. You need to create a custom Docker firewall with iptables.
There’s not a lot wrong with Ubuntu 20.04. So it’s pretty easy to fix the one thing that’s missing! Hint: Install the Cinnamon Desktop.
When testing Docker and iptables I stumbled upon something interesting. It appears Docker uses the iptables INPUT chain in an undocumented way. Well that’s interesting..