merge branch 'pr-1158'

Closes #1158
LGTMs: @hqhq @cyphar

checkpoint.go

@@ -34,6 +34,9 @@ checkpointed.`,
 		cli.StringSliceFlag{Name: "empty-ns", Usage: "create a namespace, but don't restore its properies"},
 	},
 	Action: func(context *cli.Context) error {
+		if err := checkArgs(context, 1, exactArgs); err != nil {
+			return err
+		}
 		container, err := getContainer(context)
 		if err != nil {
 			return err