07 — Write docker-compose.yml for local dev (app only)

Category: Docker Compose · Priority: High · Step: 8

Define the service, map ports, set env vars via a .env file. Use build: . to build from the local Dockerfile.

🎯 What is this task?

So far you've been running Docker with long docker run commands. Docker Compose lets you define everything in a single docker-compose.yml file and start it all with one command: docker compose up. This is how developers run apps locally in a real team.

🤖 AI Prompt

"Create a docker-compose.yml file for a Node.js Express app. The service should be called app, build from the local Dockerfile, map port 3000, and read environment variables from a .env file. Use Docker Compose v3.8 format. Add comments explaining each section."

📋 Step-by-step

Step 1 — Create a .env file in your project root

This file holds your local environment variables. Never commit this to Git.

PORT=3000
NODE_ENV=development

Step 2 — Create docker-compose.yml

version: "3.8"

services:
  app:
    # Build the image from the Dockerfile in the current folder
    build: .

    # Map port 3000 on your computer to port 3000 in the container
    ports:
      - "3000:3000"

    # Load environment variables from the .env file
    env_file:
      - .env

    # Restart the container automatically if it crashes
    restart: unless-stopped

Step 3 — Start the app

docker compose up

This builds the image (if not already built) and starts the container. You'll see logs directly in your terminal. To run it in the background:

docker compose up -d

Step 4 — Check it works

Open http://localhost:3000 — should respond.

Step 5 — Useful Compose commands

docker compose up -d       # Start in background
docker compose logs -f     # Watch logs
docker compose down        # Stop and remove containers
docker compose ps          # See running containers
docker compose build       # Rebuild the image

✅ Done when

• docker-compose.yml exists in your project root
• .env file exists (and is in .gitignore!)
• docker compose up starts the app
• http://localhost:3000 responds