add configuration to use react-express-mongo sample with Docker Dev Environments feature (#271)

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

react-express-mongodb/compose.yaml

@@ -1,6 +1,8 @@
 services:
   frontend:
-    build: frontend
+    build:
+      context: frontend
+      target: development
     ports:
       - 3000:3000
     stdin_open: true
@@ -17,7 +19,9 @@ services:
   backend:
     container_name: backend
     restart: always
-    build: backend
+    build:
+      context: backend
+      target: development
     volumes:
       - ./backend:/usr/src/app
       - /usr/src/app/node_modules