Environment Variables & Configuration - Docker & DevOps Tutorial | TechLead

Configuration in Docker

Applications should be configured through the environment, not hardcoded values. This follows the 12-Factor App methodology and makes your containers portable across development, staging, and production environments.

Setting Environment Variables

In docker run

# Individual variables
docker run -e NODE_ENV=production -e PORT=3000 myapp

# From a file
docker run --env-file .env myapp

# From host environment
export API_KEY=sk-123
docker run -e API_KEY myapp  # Passes host's API_KEY

In Dockerfile

# Set default values (can be overridden at runtime)
ENV NODE_ENV=production
ENV PORT=3000

# Use build arguments for build-time variables
ARG VERSION=1.0.0
ENV APP_VERSION=${VERSION}

# ❌ NEVER put secrets in Dockerfile
# ENV API_KEY=sk-secret-key

In Docker Compose

# docker-compose.yml
services:
  api:
    build: .
    environment:
      # Direct values
      NODE_ENV: production
      PORT: "4000"

      # From .env file variables
      DATABASE_URL: postgres://${DB_USER}:${DB_PASS}@db:5432/${DB_NAME}
      REDIS_URL: redis://redis:6379

    # Load from .env file
    env_file:
      - .env
      - .env.local    # Overrides (not committed to git)

The .env File

# .env — Docker Compose auto-loads this
NODE_ENV=production
PORT=4000

# Database
DB_USER=admin
DB_PASS=supersecret
DB_NAME=myapp
DB_HOST=db
DB_PORT=5432

# External services
REDIS_URL=redis://redis:6379
API_KEY=sk-your-api-key

# App settings
LOG_LEVEL=info
CORS_ORIGIN=https://myapp.com

⚠️ Security Warning

Never commit .env files with real secrets to version control!

# .gitignore
.env
.env.local
.env.production
*.pem

Instead, commit a .env.example with placeholder values:

# .env.example — commit this
NODE_ENV=development
DB_USER=admin
DB_PASS=change-me
API_KEY=your-api-key-here

Multi-Environment Configuration

# docker-compose.yml (base)
services:
  api:
    build: .
    env_file: .env

# docker-compose.dev.yml
services:
  api:
    env_file:
      - .env
      - .env.development
    volumes:
      - ./src:/app/src

# docker-compose.prod.yml
services:
  api:
    env_file:
      - .env
      - .env.production
    restart: always

# Run with specific environment
docker compose -f docker-compose.yml -f docker-compose.dev.yml up
docker compose -f docker-compose.yml -f docker-compose.prod.yml up -d

Accessing Environment Variables in Code

// Node.js
const config = {
  port: parseInt(process.env.PORT || '3000'),
  nodeEnv: process.env.NODE_ENV || 'development',
  databaseUrl: process.env.DATABASE_URL,
  redisUrl: process.env.REDIS_URL,
  apiKey: process.env.API_KEY,
};

// Validate required variables
const required = ['DATABASE_URL', 'API_KEY'];
for (const key of required) {
  if (!process.env[key]) {
    console.error(`Missing required environment variable: ${key}`);
    process.exit(1);
  }
}

Key Takeaways

✅ Use environment variables for all configuration — never hardcode
✅ Use .env files for local development
✅ Never commit secrets to version control
✅ Validate required environment variables at startup
✅ Use different compose override files for each environment