[CentOS] C7 and docker storage

Was working on docker on a server, and on startup, I see
Nov 29 10:58:27 <servername> dockerd-current:
time="2017-11-29T10:58:27.612849959-05:00" level=warning msg="devmapper:
Usage of loopback devices is strongly discouraged for production use.
Please use `--storage-opt dm.thinpooldev` or use `man docker` to refer to
dm.thinpooldev section."
Nov 29 10:58:27 <servername> dockerd-current:
time="2017-11-29T10:58:27.655600686-05:00" level=warning msg="devmapper:
Base device already exists and has filesystem xfs on it. User specified
filesystem  will be ignored."

The latter would explain the message my user's job gave him when it tried
to umount /

A bit of googling, and I see something called overlayFS can be used... but
I know nothing about that, or how dangerous it is.  Anyone got a pointed
to something more than the minimal how to configure docker to use it?

        mark