> I briefly looked at caddy 2 - but wasn't able to find any out-of-the box tricks for listening to docker messages and dynamically configure sites in a sane way.

Like this? (Am not a Docker user, but I know this is an insanely popular solution) https://github.com/lucaslorentz/caddy-docker-proxy

There's also a WIP ingress controller: https://github.com/caddyserver/ingress/