Allow volumes from to be individual files

Fixes #4741
Right now volumes from expected a dir and not a file so when the drivers
 tried to do the bind mount, the destination was a dir, not a file so it
 fails to run.
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Add integration test for volumes-from as file
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #4752 from crosbymichael/fix-volumes-from-files

Allow volumes from to be individual files