hmm odd you def shouldnt have to do that, you would of course need to pull down the latest image but that wont require a manual deletion of your container. In any case, great you have it running, just a quick word of warning, watch out for this bug:- https://github.com/jfurrow/flood/issues/175
So you need to make sure you define the downloads folder otherwise you will find the torrent just sits there at 0KB/s and wont download.
Had to do the same. Tried just adding the port and variable and restarted a few times. Ended up just removing it and reloading it via the Add Docker on the Docker page.