How about sqlar as a container format? https://sqlite.org/sqlar.html A regular sqlite database file, with anything you like in it. Mountable as a file system with sqlarfs. Written by the sqlite guy.
Interesting I didn't know this existed. Is there a way to layer sqlar like docker images? (Besides just tarring them up I guess.)
I wonder if this could be implemented with the WAL/journal system. Make each layer immutably append to the previous layers to make restarting at any layer trivial. I'm not sure if there's such a way to hook into the journal directly like that though.
Should be doable with overlayfs (or similar) or alternatively some extensions to sqlar.
sqlar is after all only a table definition, if you don't need FUSE access or are willing to write your own, SQLite3 can go a long way of providing arbitrary neat functionality.
