Make volume.Containers private

Also wrap access in mutex.
Makes sure we don't have any pontential for races in accessing this.
It also doesn't really need to be/shouldn't be in the config.json anyway

Docker-DCO-1.1-Signed-off-by: Brian Goff <bgoff@cpuguy83-mbp.home> (github: cpuguy83)
Merge pull request #8372 from cpuguy83/change_volume_containers_to_private

Make volume.Containers private