Yeah, configure `ufw default deny incoming` and Docker will sneakily bypass that and gives the internet unfettered access to your ElasticSearch and MongoDB and whatever. And somehow, that's apparently "not a bug". It's one of the biggest footguns I know of, and could not believe it was expected behavior when I first encountered it.

We have a strict policy that servers must not have a routable IP address at all, and must have an external firewall applied. It's a good idea in any case, but turns out it's absolutely necessary with Docker.