It’s no different in Virtualmin (vs virtual hosting Discourse behind Apache on a system without Virtualmin). You’d proxy to the Docker-contained server running it.
This is a reasonable discussion on the topic (be careful if doing additional searches, the top result when searching for this is an insane guide that proxies through both Apache and HAProxy, which is a ridiculous thing to do):
nginx is pretty similar, if you’re using it. I’m sure there’s docs for that, too. But, as with Apache, if you see a crazy bunch of stuff in the mix, it’s probably wrong. This is not complicated and doesn’t need a bunch of extra software! Apache (or nginx) can proxy to a Docker container very easily with no additional stuff! Anything that suggests nginx+Apache is also crazy and senselessly complicated. Just Apache and mod_proxy will do the thing, assuming Docker is configured to expose the app on a local file socket (a local port would also work, but on a shared virtual hosting system, file sockets are safer, because a file, or rather the directory the file is in, can be owned by the user that owns the app).