UnmountVolumes used to also unmount 'specialMounts' but it no longer does after
a recent refactor of volumes. This patch corrects this behavior to include
unmounting of `networkMounts` which replaces `specialMounts` (now dead code).
Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Upstream-commit: 04f99a6ca8232e43169b9a0706e435c551c798a3
Component: engine