docker-compose node app can't reach mongo - node.js

Question

I'm having a weird connectivity issue with my docker containers.
This is my compose.yaml
version: "3.3"
services:
node-app:
image: "${CI_REGISTRY_IMAGE}:${CI_COMMIT_SHA}"
container_name: "node-app"
depends_on:
- mongo
env_file: ./config/.env
ports:
- 43332:43332
networks:
- app-network
environment:
WAIT_HOSTS: mongo:27017
mongo:
container_name: "app-mongo"
image: mongo:4.0
volumes:
- /data/db:/data/db
ports:
- 27017:27017
networks:
- app-network
networks:
app-network:
So I'm trying to set up a CI/CD pipeline and the first thing it does is to run tests when code is pushed. The pipeline runs this command docker-compose run --rm app-node npm run test.
However the it keeps getting the following error.
MongoNetworkError: failed to connect to server [mongo:27017] on first connect [MongoNetworkError: connection 0 to mongo:27017 timed out]
My app is connecting to mongo like this
mongoose.connect('mongodb://mongo:27017', { useNewUrlParser: true });
mongoose.Promise = global.Promise;
mongoose.connection.on('error', error => {
logMe(error);
process.exit(1);
});
The weird thing is I can see that the app container itself can talk to the mongo conatiner by its service name ('mongo') at the OS level.
So if I run this - docker-compose run --rm app-node ping -c 1 mongo, the ping is successful.
It's running under the following environment
$ docker -v
Docker version 18.09.7, build 2d0083d
$ docker-compose version
docker-compose version 1.24.1, build 4667896b
docker-py version: 3.7.3
CPython version: 3.6.8
OpenSSL version: OpenSSL 1.1.0j 20 Nov 2018
What could I be doing wrong?? Any help is appreciated.