Linuxserver already had an image for watcher3, https://github.com/linuxserver-archive/docker-watcher-stale, but it was archived because developer didn’t help with docker issues, he wasn’t receptive about suggestions.
Project seems to be stuck, but there is a new fork now, trying to move project forward, https://github.com/barbequesauce/Watcher3 at GitHub, and they have a link to docker image in readme, although it didn’t worked to me, I think maintainer will be receptive to changes to improve docker support. It would be nice to reopen archived image, and change it to use this new fork. To solve issue with posters I added --posters /config/posters to run command, as they did in their docker image.