Installing docker on an already running web server?

The recommended setup in this situation is to have nginx running on 80/443 in front, proxying to Discourse and your other sites as needed.

If you can move your PHP files over to php-fpm and not have to go through apache, that’ll be a neater setup than nginx → {apache, discourse}.

This topic has more information:

I would like to heavily recommend against putting apache in front. Without the evented stuff, it’ll fill up the workers with the notification polling.

As for installing in a subdirectory, that existed in a broken form a time ago, then it was ripped out, and there’ve been people asking for it again but it’s tricky due to all the URL generation, and the proxy switching esp. if they’re not on the same machine, and… it’s not available right now.