Docker bind to a project folder (common for development) allows the image to take over the shell. Taking over the shell allows taking over the package manager. That allows taking over the system.

I keep spreadsheets in project folders. Allowing full writes to a project folder allows taking over the shell. See above.

Folder-level permissions are not sufficient. Sorry, I shortened it too much, but basically - there's almost no practical difference between allowing writes to a dev project folder and allowing writes everywhere. (It would stop some really trivial attempts) Your actual system folders are safe of course... but who cares about those really?