add docker network prune

`docker network prune` prunes unused networks, including overlay ones.
`docker system prune` also prunes unused networks.

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
Upstream-commit: 7e24c16086a9a4f38e241e51837f2be4877c04a6
Component: engine

components/engine/cli/command/system/prune.go

@@ -39,6 +39,7 @@ const (
 	warning = `WARNING! This will remove:
 	- all stopped containers
 	- all volumes not used by at least one container
+	- all networks not used by at least one container
 	%s
 Are you sure you want to continue?`
 
@@ -64,13 +65,14 @@ func runPrune(dockerCli *command.DockerCli, opts pruneOptions) error {
 	for _, pruneFn := range []func(dockerCli *command.DockerCli) (uint64, string, error){
 		prune.RunContainerPrune,
 		prune.RunVolumePrune,
+		prune.RunNetworkPrune,
 	} {
 		spc, output, err := pruneFn(dockerCli)
 		if err != nil {
 			return err
 		}
-		if spc > 0 {
-			spaceReclaimed += spc
+		spaceReclaimed += spc
+		if output != "" {
 			fmt.Fprintln(dockerCli.Out(), output)
 		}
 	}