In that situation, you cannot use the discourse-setup script. Copy app.yml from samples to containers and fill in the values manually.
The tutorial includes changes to this file, which will ensure that Discourse can start up (listening on a socket).
After that, you can install nginx on your host and set it up to forward the subdomain for Discourse as explained in the tutorial. Your Apache installation will have to move to a different port so nginx can start up, and nginx will need some configuration to forward everything else to Apache as needed.
Any alternative using Apache directly as your front end HTTP server will have some performance issues because Apache deals poorly with long polling connections used by message bus.