I have no idea about cozy.io, but just to offer another option, I’ve been running Seafile for years and it’s pretty solid piece of hardware. And while it does have other stuff than just file storage/sharing, it’s mostly about just files and nothing else. Android client isn’t the best one around, but gets the job done (background tasks at least on mine tend to freeze now and then), on desktop it just works.
Not that it’s really relevant for the discussion, but yes. You can do that, with or without chroot.
That’s obviously not the point, but we’re already comparing oranges and apples with chroot and containers.