I have been having this issue too, I was talking with someone at work who uses docker on a Synology and and he uses watchtower to update his containers. Watchtower checks the images that are installed and automatically downloads shuts down installs and restarts the container. I tried installing it and there's no config needed. The next morning I checked and all the containers that needed an update said they were created 2 hours ago and started 2 hours ago. The only downside is that because of the way unraid tracks when containers are updated they all still show that they need an update. When clicking on update it shows 0b downloaded, restarts the container and then it's good.