How does the command key in docker-compose file work

Question

I am trying to understand the docker sample application 'example-voting-app'. I am trying to build the app with docker-compose. I am confused with the behaviour of 'command' key in docker compose file and the CMD Instruction in Dockerfile. The application consists of a service called 'vote'. The configuration for the vote service in docker-compose.yml file is:

services: # we list all our application services under this 'services' section.
  vote:  
    build: ./vote # specifies docker to build the 
    command: python app.py
    volumes:
     - ./vote:/app
    ports:
      - "5000:80"
    networks:
      - front-tier
      - back-tier

The configuration of the Dockerfile provided in ./vote directory is as below:

# Using official python runtime base image
FROM python:2.7-alpine

# Set the application directory
WORKDIR /app

# Install our requirements.txt
ADD requirements.txt /app/requirements.txt
RUN pip install -r requirements.txt

# Copy our code from the current folder to /app inside the container
ADD . /app

# Make port 80 available for links and/or publish
EXPOSE 80

# Define our command to be run when launching the container
CMD ["gunicorn", "app:app", "-b", "0.0.0.0:80", "--log-file", "-", "--access-logfile", "-", "--workers", "4", "--keep-alive", "0"]

My doubt here is which command ( 'python app.py' or 'gunicorn app:app -b ...') will be executed when i try building the application using docker-compose up

Answer 1

The Docker Compose command:, or everything in a docker run invocation after the image name, overrides the Dockerfile CMD.

If the image also has an ENTRYPOINT, the command you provide here is passed as arguments to the entrypoint in the same way the Dockerfile CMD does.

For a typical Compose setup you shouldn't need to specify a command:. In a Python/Flask context, the most obvious place it's useful is if you're also using a queueing system like Celery with the same shared code base: you can use command: to run a Celery worker off of the image you build, instead of a Flask application.