I cannot establish connection between two containers in Heroku - node.js

Question

I have a web application built using Node.js and MongoDB. I have containerized the app using Docker and it was working fine locally but once i have tried to deploy it to production I couldn't establish connection between the backend and MongoDB container. For some reason the environment variables are always undefined.
Here is my docker-compose.yml:
version: "3.7"
services:
food-delivery-db:
image: mongo:4.4.10
restart: always
container_name: food-delivery-db
ports:
- "27018:27018"
environment:
MONGO_INITDB_DATABASE: food-delivery-db
volumes:
- food-delivery-db:/data/db
networks:
- food-delivery-network
food-delivery-app:
image: thisk8brd/food-delivery-app:prod
build:
context: .
target: prod
container_name: food-delivery-app
restart: always
volumes:
- .:/app
ports:
- "3000:5000"
depends_on:
- food-delivery-db
environment:
- MONGODB_URI=mongodb://food-delivery-db/food-delivery-db
networks:
- food-delivery-network
volumes:
food-delivery-db:
name: food-delivery-db
networks:
food-delivery-network:
name: food-delivery-network
driver: bridge

Answer 1

This is expected behaviour:
Docker images run in dynos the same way that slugs do, and under the same constraints:
…
Network linking of dynos is not supported.
Your MongoDB container is great for local development, but you can't use it in production on Heroku. Instead, you can select and provision an addon for your app and connect to it from your web container.
For example, ObjectRocket for MongoDB sets an environment variable ORMONGO_RS_URL. Your application would connect to the database via that environment variable instead of MONGODB_URI.
If you'd prefer to host your database elsewhere, that's fine too. I believe MongoDB Atlas is the official offering.