Support STOPSIGNAL instruction in commit change

Signed-off-by: Stéphane Este-Gracias <sestegra@gmail.com>

builder/dockerfile/builder.go

@@ -35,6 +35,7 @@ var validCommitCommands = map[string]bool{
 	"expose":      true,
 	"label":       true,
 	"onbuild":     true,
+	"stopsignal":  true,
 	"user":        true,
 	"volume":      true,
 	"workdir":     true,