I was encountering the 'Cannot openTUN /TAPdev /dev/net/tun' error, which I was finally able to resolve by following the Configuration section of https://www.kernel.org/doc/Documentation/networking/tuntap.txt these instructions on the unraid host OS and then adding --device /dev/net/tun to the Extra Parameters when setting up the container.
During this process I also turned off Privileged mode for the container and added --cap-add=NET_ADMIN to the Extra Parameters. My understanding is that this is more restrictive than enabling full Privileged mode, but still allows the VPN to function.
I'm now able to use the container without privileged mode enabled, and have been using qbittorrent without issue. If my understanding is correct, it may be worthwhile to change the template for the docker to remove privileged mode, as it does open some security concerns, and use the NET_ADMIN setting instead.