add configuration to use react-express-mysql sample with Docker Dev Environments feature (#270)

Signed-off-by: Guillaume Lours <guillaume.lours@docker.com>

react-express-mysql/compose.yaml

@@ -4,6 +4,7 @@ services:
       args:
       - NODE_ENV=development
       context: backend
+      target: development
     command: npm run start-watch
     environment:
       - DATABASE_DB=example