Correct manifest push without content trust

The script expects an empty second argument to mean no trust and anything else
to mean trust.

Signed-off-by: Ian Campbell <ijc@docker.com>

src/cmd/linuxkit/pkglib/docker.go

@@ -75,7 +75,7 @@ func (dr dockerRunner) pushWithManifest(img, suffix string) error {
 		return err
 	}
 
-	dctArg := "0"
+	var dctArg string
 	if dr.dct {
 		dctArg = "1"
 	}