Well, I checked this morning as i've only realy started playing with Sonarr and the WebUI wasn't working, I have AutoUpdates turned on for docker containers, so I just assumed there was an update, and came to this thread to see if there was a 'known issue'.
Changed the repository to binhex/arch-sonarr:latest, and pulled the new container. Then I bashed in and noticed the Sonarr.exe difference. Changed this, and Sonarr V3 started. Then I thought, oh well i've changed this now but as soon as another update is published I'm going to have to do it again. So deleted the container from Unraid and created a new one from a template, using :latest again, and encountered the same issue.
Don't know if that's useful?