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.