Consider fuse-overlayfs unprivileged overlays

[dtrudg]

Is your feature request related to a problem? Please describe.

Unprivileged execution of containers cannot use overlays. There is a fuse-overlayfs project that allows FUSE mounted overlayfs where kernel support is not available:

https://github.com/containers/fuse-overlayfs

Describe the solution you'd like

Consider whether to implement fuse-overlayfs based support for unprivileged overlayfs. May not be applicable depending on the rate of deployment of native kernel support. See #818