This moves the platform specific stuff in a separate package and keeps
the `volume` package and the defined interfaces light to import.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
In some cases a mount spec would not be properly backported which could
lead to accidental removal of the underlying volume on container remove
(which should never happen with named volumes).
Adds unit tests for this as well. Unfortunately I had to add a daemon
depdency for the backport function due to looking up `VolumesFrom`
specs.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
