Presently the container overlays are stored in /usr/lib/live/mount/persistence/container/storage/overlay on the main storage. When using several containers (or several version of the same container config) this directory can grow very large.

Allow using a different path for storing the overlay data (potentially a separate partition/device; see https://vyos.dev/T5441 for a request to add persistently-mounted storage to a VyOS system.)