The dockers autoupdate based on what is available to download from the Plex website:
https://plex.tv/downloads/1/archive
I'm a little confused here. Plex notified me in the interface - I hadn't visited the forum to find out about an update. And the archive page you've linked to has version 0.9.12.8, whereas the docker has version 0.9.12.4.
Granted, it does say "Please install manually" and I can click that text do download 0.9.12.8..., but I have no idea what I should do with that file (if anything)...?