Add STOPSIGNAL instruction to dockerfiles.

This way, images creators can set the exit signal their programs use.

Signed-off-by: David Calavera <david.calavera@gmail.com>
Upstream-commit: 3781cde61ff10b1d9114ae5b4c5c1d1b2c20a1ee
Component: engine

components/engine/builder/parser/parser.go

@@ -61,6 +61,7 @@ func init() {
 		command.Entrypoint: parseMaybeJSON,
 		command.Expose:     parseStringsWhitespaceDelimited,
 		command.Volume:     parseMaybeJSONToList,
+		command.StopSignal: parseString,
 	}
 }