How to completely destroy Mongo database created by docker-compose?

I am using docker-compose to create a MongoDB service. Here is how it looks:

    image: 'mongo'
      - MONGO_DATA_DIR=/data/db
      - ./container-scripts/mongo-init.js:/docker-entrypoint-initdb.d/mongo-init.js
      - ./data/db:/data/db

I build and run the service using sudo docker-compose up --build -d database and this creates the MongoDB container.

During testing I add a few collections to the database by connecting to the database through mongo <container ip address> and issuing a few mongo commands (e.g. db.createCollection())

Afterwards, I use sudo docker-compose down -v to remove the container, and I also run sudo docker system prune -a -f just to be sure. I expect that the next time I build and run the image, the database should be empty (i.e. have no collections), however I can see the collections are still there. Why? How do I get a clean MongoDB container?

Source: StackOverflow