Docker: Unhandled Promise rejection when trying to access mongoDB using docker - node.js

Question

So I was trying to deploy my very basic Node.js application using docker on a remote linux server(digital ocean droplet) following this tutorial. I'm pretty new to deployment related stuff so i'm surely doing some basic error. Please bear with me for the long post.
The code for project and docker, docker-compose config files that I was using was:
.env
MONGO_USERNAME=sammy
MONGO_PASSWORD=password
MONGO_DB=sharkinfo
PORT=8080
MONGO_PORT=27017
MONGO_HOSTNAME=127.0.0.1
Dockerfile
FROM node:10
# Create app directory
WORKDIR /usr/src/app
# Install app dependencies
# A wildcard is used to ensure both package.json AND package-lock.json are copied
# where available (npm#5+)
COPY package*.json ./
RUN npm install
# If you are building your code for production
# RUN npm ci --only=production
# Bundle app source
COPY . .
EXPOSE 8080
CMD [ "node", "src/index.js"]
docker-compose.yml
version: '3'
services:
nodejs:
build: .
env_file: .env
environment:
- MONGO_USERNAME=$MONGO_USERNAME
- MONGO_PASSWORD=$MONGO_PASSWORD
- MONGO_HOSTNAME=myDB
- MONGO_PORT=$MONGO_PORT
- MONGO_DB=$MONGO_DB
ports:
- "80:8080"
networks:
- app-network
myDB:
image: mongo:4.1.8-xenial
env_file: .env
environment:
- MONGO_INITDB_ROOT_USERNAME=$MONGO_USERNAME
- MONGO_INITDB_ROOT_PASSWORD=$MONGO_PASSWORD
volumes:
- dbdata:/data/db
networks:
- app-network
networks:
app-network:
driver: bridge
volumes:
dbdata:
mongoose.js
const mongoose = require('mongoose');
const {
MONGO_USERNAME,
MONGO_PASSWORD,
MONGO_HOSTNAME,
MONGO_PORT,
MONGO_DB
} = process.env;
const url = `mongodb://${MONGO_USERNAME}:${MONGO_PASSWORD}#${MONGO_HOSTNAME}:${MONGO_PORT}/${MONGO_DB}?authSource=admin`;
mongoose.connect(url, {
useNewUrlParser: true,
useCreateIndex: true,
useFindAndModify: false
});
Then I created and ran a docker image using the commands (in-order) - docker-compose build docker-compose up -d.
Both the node and mongoDB server show up in list of running docker processes. However when i do docker logs node-application, i get an error:
Server is up on port 8080
(node:1) UnhandledPromiseRejectionWarning: MongoNetworkError: failed to connect to server [mydb:27017] on first connect [MongoNetworkError: connect ECONNREFUSED 172.19.0.3:27017]
at Pool.<anonymous> (/usr/src/app/node_modules/mongodb-core/lib/topologies/server.js:564:11)
at Pool.emit (events.js:198:13)
at Connection.<anonymous> (/usr/src/app/node_modules/mongodb-core/lib/connection/pool.js:317:12)
at Object.onceWrapper (events.js:286:20)
at Connection.emit (events.js:198:13)
at Socket.<anonymous> (/usr/src/app/node_modules/mongodb-core/lib/connection/connection.js:246:50)
at Object.onceWrapper (events.js:286:20)
at Socket.emit (events.js:198:13)
at emitErrorNT (internal/streams/destroy.js:91:8)
at emitErrorAndCloseNT (internal/streams/destroy.js:59:3)
at process._tickCallback (internal/process/next_tick.js:63:19)
(node:1) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:1) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
I've tried various tweaks to my code with no success and it continues to give the same error. What could be the issue here?