Thanks very much for the reply.
Ah, sorry I was not clear, it was only the Tautulli’s that I was planning to run in docker containers, at least for now (1 for each of my PMS)
The PMS on the nas is running on the nas, not in a container.
But… anyways, I got it to work. 
So for anyone else that might find it useful.
When you initially create the container, in the Network tab, there is a checkbox to use the same network as Docker host, so I checked that.
(fwiw, You can only do this when creating the container, there is no Network tab when you edit an existing container)
Once you select that you can no longer change anything in the Port Settings tab, so I just carried on and set the environment vars.
Launched the container and loaded Tautulli from the ip address of the nas and port 8181
and… success Tautulli loaded and now can verify the server using the ip address of the nas and port 33400.
(Once you have finished setting up the container and look in the docker network panel in the DSM you will see the container has been added to the host network section)
