As for how will you know if there is an update, Plex usually tells me when I open its webUI if there is an update. When that happens, I just close it and restart the docker to get the new version. Other applications will be different, which is why there is no way for unRAID to know, no consistent way to tell with all possible applications. NZBGet in its webUI has a way to check versions, for example, but you have to tell it to check.