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/.docker/docker-compose.yaml

@@ -0,0 +1,43 @@
+services:
+  frontend:
+    build:
+      context: frontend
+      target: dev-envs
+    ports:
+      - 3000:3000
+    stdin_open: true
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock
+    restart: always
+    networks:
+      - react-express
+    depends_on:
+      - backend
+
+  backend:
+    restart: always
+    build:
+      context: backend
+      target: dev-envs
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock
+    depends_on:
+      - mongo
+    networks:
+      - express-mongo
+      - react-express
+    expose: 
+      - 3000
+  mongo:
+    container_name: mongo
+    restart: always
+    image: mongo:4.2.0
+    volumes:
+      - ./data:/data/db
+    networks:
+      - express-mongo
+    expose:
+      - 27017
+networks:
+  react-express:
+  express-mongo: