Redis – If I am using docker-compose.yml why would I still need a Dockerfile – an if so, how many?

Let’s say I have a docker-compose.yml like further below.

When I fire up docker-compose up it works.

I am wondering of what use the Dockerfile could possibly be.

The reason why I am asking is this section of the documenation:

https://docs.docker.com/compose/

Using Compose is basically a three-step process:

• Define your app’s environment with a Dockerfile so it can be reproduced anywhere.
• Define the services that make up your app
  in docker-compose.yml so they can be run together in an isolated environment.
• Run docker-compose up and Compose starts and runs your entire app.

I my case I haven’t created a Dockerfile, but still I can reproduce that container everywhere.

So, what is the purpose of the Dockerfile in conjunction or as opposed to docker-compose.yml?

And then I’d also like to ask this intertwined follow up question?

If there is useful application of a Dockerfile despite having already provide a docker-compose.yml, Would I have to use two Dockerfiles since I have two applications in one container?

I am confused.

version: "3.8"

services:
  mynodeapp:
    image: node:15.8.0
    container_name: myappname
    working_dir: /app
    volumes:
      - .:/app
      - ~/.ssh:/root/.ssh
    expose:
      - 3000
    restart: always
    ports:
      - "3000:3000"
    networks:
      - backend
    environment:
      HOST: 0.0.0.0
      NODE_ENV: $NODE_ENV
  redis:
    container_name: myredis
    networks:
      - backend
    image: redis
    ports:
      - "6379:6379"
    volumes:
      - $PWD/redis-data:/var/lib/redis
      - $PWD/redis.conf:/usr/local/etc/redis/redis.conf
 networks:
   backend:
     driver: ${DOCKER_NETWORK_DRIVER}