Docker/Plex without --net=host
I just moved my server over to a linux/docker container (specifically linuxserver.io) and was having a hard time getting things to work the way I wanted. I did not want to use --net=host, so I setup just the ports. After starting up the container, I could not get Plex to detect server. I could see Plex running when I went to Linux.IP:32400/web, but it would not detect the server and show my library.
I was finally able to access the server by setting up a proxy tunnel and configuring my browser to use it. Once in, I noticed that the local IP address was reporting the docker container IP 172.17.X.X. I found that if I added the IP address of my Linux server hosting Docker to the "Custom server access URLs" field, my problem was solved.
You should be able to get away without having to tunnel by putting customConnections=" https://192.168.X.X:32400" (Server hosting Docker) in your preferences.xml. After restarting it took a few minutes for this address to publish to Plex. After that, everything worked great. I'm running Plex in Docker without --net=host.