point docker container to a host web server (nginx)

So I'm trying to install a GitLab instance inside my host. My host already have Nginx installed (without docker) and has other subdomains running fine.

My goal is to have them both running (if possible) but GitLab's docker container cant run while the host's web server (Nginx) is on, and the other way around. I end up with a docker error message below:

Error starting userland proxy: listen tcp 0.0.0.0:80: bind: address already in use.

Or at some point the port 443, and that's the Nginx ports running.My question is how do I point my container to run through my installed Nginx on my virtual host so I can visit my GitLab instance as a subdomain?My server is running ubuntu 16.04.