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Docker Compose File

The heart of Docker Compose is the docker-compose.yml file. This YAML file defines all the services, networks, and volumes for your application. Understanding its structure is key to using Compose effectively.

Compose File Structure

A typical docker-compose.yml has three main top‑level sections:
  • services: Defines the containers (each service becomes a container).
  • networks: (Optional) Custom networks for services.
  • volumes: (Optional) Named volumes for persistent data.

Basic Service Definition

Here’s a simple service using an image:
services:
  web:
    image: nginx:latest
    ports:
      - "8080:80"
    environment:
      - NGINX_HOST=localhost
    volumes:
      - ./html:/usr/share/nginx/html
  • image: The Docker image to use.
  • ports: Maps host ports to container ports (like -p).
  • environment: Sets environment variables.
  • volumes: Mounts volumes (bind mounts or named volumes).

Building an Image from a Dockerfile

Instead of an existing image, you can build an image from a Dockerfile:
services:
  app:
    build: ./app
    ports:
      - "3000:3000"
build points to a directory containing a Dockerfile.

Networks and Volumes in Compose

Define custom networks and volumes at the bottom:
services:
  db:
    image: postgres
    volumes:
      - db-data:/var/lib/postgresql/data
    networks:
      - backend

volumes:
  db-data:

networks:
  backend:

Versioning

Compose files usually start with a version (e.g., version: '3.8'). However, recent Docker versions support the simplified syntax without a version tag (the default uses the latest features). We'll use the modern style without explicit version.


Two Minute Drill
  • The docker-compose.yml file defines services, networks, and volumes.
  • Each service can specify image, build, ports, environment, volumes, etc.
  • Use named volumes and networks to share resources between services.
  • Compose files are YAML; indentation matters.

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