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.
is it copied from the `container`, or from the `image` - i'm kinda wondering about what happens on `docker restart` - I presume that if I've `docker stop`ed a container, then manually removed the volume, it'll be re-created, and thus re-initialised? (yes, i'm not sure if there's something that can be written to cover this, or if its just a word order thing)

and is there a way i could change the underlying container fs between restarts