Volumes tend to carry important data and pruning them on `docker system
prune` can easily cause unwanted data loss.
Let's play it safe and not prune volumes on `system prune` by default,
and instead provide an option.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
(cherry picked from commit 37fd6128dc)
Signed-off-by: Andrew Hsu <andrewhsu@docker.com>