If the port isn’t being explicitly exposed then it won’t expose by default. I have my containers set up in a similar way where nginx has it’s own network and I attach any containers I want to proxy to the same network. On most cases I’m able to remove the ports section from config and point nginx at the name of the container, and the container’s default http port. The way OP originally configured this should be fine IF nginx and audiobookshelf are on the same docker network.