I've heard about Watchtower (auto update) and DUIN (docker image update notifier), but I haven't quite found something that will "just tell me what updates are available, on a static site".

I want to "read all available updates" at my convenience, not get alerts reminding me to update my server.

Maybe I need to write some sort of plugin to DUIN that appends to a text file or web page or SQLite db... Hm.