Move volumes out of container.go and into volumes.go

This helps clean up the container file and move volumes into
one location.  We currently cannot move volumes to a sub pkg
because they depend on Container and also modify fields on the
container.
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #4159 from crosbymichael/move-volumes

Move volumes out of container.go and into volumes.go