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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}
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2

Answers


  1. 0

    In your example, you are using already built containers (node:15.8.0 / redis), that’s why docker-compose doesn’t need any Dockerfile.

    The Dockerfile is needed when you want to build your own image.
    In that case, docker-compose will need instructions on how to build your image.

    So just to summarize – if you want to use some existing container "as is" – you don’t need Docekrfile. docker-compose will pull the image from docker registry and it’ll be ready to use without any modifications. On the other hand, if you want to build your own container (e.g. building an application), you’ll need Dockerfile.

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  2. 0

    You can do more complex setup in a dockerfile. In your case there exists an image and you have no modifications to make to the image. So don’t provide one.

    Just so you know, common practice is to COPY the app files in production as opposed to mapping a volume from local machine. This would be done in a dockerfile. For each service, you can specify the build property in docker-compose.yaml with the location of the dockerfile should you wish to. Usually that is unique to the service/container.

    Also, slight terminology point of order: you have more than one container. Each service will spin up a separate container. docker ps will show you this. You have one compose file that manages several containers.

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