back-end: fix mongodbUrl

nodejs/tools/mongodb.js

@@ -4,7 +4,7 @@ if (process.env.MONGODB_USERNAME && process.env.MONGODB_PASSWORD && process.env.
     mongodbUrl = 'mongodb://' + process.env.MONGODB_USERNAME + ':' + process.env.MONGODB_PASSWORD + '@' + process.env.MONGODB_PORT_27017_TCP_ADDR + ':' + process.env.MONGODB_PORT_27017_TCP_PORT + '/' + process.env.MONGODB_INSTANCE_NAME;
 }
 else {
-    mongodbUrl = 'mongodb://localhost:27017/danmaku';
+    mongodbUrl = 'mongodb://mongo:27017/danmaku';
 }
 mongoose.connect(mongodbUrl);