Overlay for Singularity Containers
==================================

"Persistent Overlays"
https://sylabs.io/guides/3.5/user-guide/persistent_overlays.html

>   "Persistent overlay directories allow you to overlay a writable file
    system on an immutable read-only container for the illusion of
    read-write access. You can run a container and make changes, and
    these changes are kept separately from the base container image."