It looks like you've packaged nginx inside the docker and I have the RP setup as a systemd service, not in a docker to serve audiobookshelf and gonic at the moment. I can't imagine it's going to be easy to configre a second nginx instance inside a docker to work aside the systemd one.
Actually the whole reason for docker is to have an nginx and other software run independently and alongside your system install. Think of docker as a computer inside a computer.
HTH
It looks like you've packaged nginx inside the docker and I have the RP setup as a systemd service, not in a docker to serve audiobookshelf and gonic at the moment. I can't imagine it's going to be easy to configre a second nginx instance inside a docker to work aside the systemd one.