The docker-compose.yml file

When you run your project (with docker-compose up or other commands), Docker Compose will use this file to orchestrate its components.

Below we’ve highlighted the relevant parts of the file - you don’t need to go through it all right now, but you will probably want to come back to it later.

Services or containers

services:
  web:
    [...]
  db:
    [...]

The file defines two services - in effect, two containers that Docker Compose will launch. One is the web service or container, containing your Django project. The other is the db container, our local stand-in for the database cluster that would be available on the Cloud.

The web service

build: "."

Docker Compose needs to know where to find the instructions to build the image for this container - the answer is right here, because that’s where the Dockerfile is.

command: python manage.py runserver 0.0.0.0:80

When the container starts up, start the Django runserver on port 80.

ports:
  - "8000:80"

… and map that port 80 inside the container to port 8000 outside.

env_file: .env-local

Read the file .env-local for any environment variables.

links:
  - "db:postgres"

Resolve the hostname postgres to the db container.

The db service

image: postgres:9.6-alpine

This container is based on a pre-built, off-the-shelf image, one specifically created to run Postgres 9.6 on Alpine Linux.

environment:
  POSTGRES_DB: "db"

When the container is launched, its environment should include the POSTGRES_DB variable with the value db.