Proposal: Copy data from container to empty vol in all cases

Currently data is only copied into a volume if it is a _normal_ volume,
e.g. not for volumes-from or host-mounts.

At one point a host-mounted volume would not be automatically created if
the dir didn't exist already, today that is no longer true.  If we are
willing to create the dir, we might as well populate it too.

This makes it so data is always copied if the volume is empty.
It copies from the contianer's fs.
Volumes do not get re-initialized: https://github.com/docker/docker/blob/master/daemon/volumes.go#L80-L82