Well, that was a fun few hours.
During configuration of the docker container, I must have accidently set ENABLE_FLOOD to yes. When I started the container I could not connect to the WebUI no matter what I tried. Searched the forum thread but didn't see anyone else with the same problem. Privoxy and VPN were working okay so I dug into the container and discovered that Nginx hadn't even started. Found the "/home/nobody/rutorrent.sh" script and where it disables the webui if ENABLE_FLOOD == yes.
Up to this point, I didn't realise there was an alternative webui for the container called Flood . I thought ENABLE_FLOOD was a network setting to protect against some kind of flood attack, TCP_SYN, or something. Also, when it is running, clicking on the "WebUI" context menu option for the container under the Docker tab still sends you to http://x.x.x.x:9080/ which fails because of course the normal webui is not running on this port. The alternative Flood ui is running on port 3000.
I did not see this information mentioned anywhere on the thread or docker description, so I'm posting it here for anyone else who may also be a first-time user of this particular docker container.