Environment Variables in Docker Compose

Managing environment variables effectively is crucial for configuring and deploying containerized applications using Docker Compose. This blog post provides a comprehensive guide to understanding and utilizing environment variables within your Compose setups.

Defining Environment Variables

You can define environment variables in several ways:

• .env files: Create a .env file in the same directory as your docker-compose.yml file. Variables defined here are loaded by default.

# .env file
POSTGRES_USER=example
POSTGRES_PASSWORD=password

• docker-compose.yml: Define variables directly within your docker-compose.yml file under the environment key for a specific service.

version: '3.8'
 
services:
  web:
    image: nginx:latest
    environment:
      VIRTUAL_HOST: example.com
      VIRTUAL_PORT: 80

• Shell environment: Pass variables directly from your shell when running docker-compose commands.

DATABASE_URL=postgres://user:password@host:port/database docker-compose up -d

• Environment files: Specify external environment files using the env_file key. These files follow the same format as .env files.

version: '3.8'
 
services:
  db:
    image: postgres:latest
    env_file:
      - db.env

Variable Substitution

Leverage variable substitution to define dynamic values based on other environment variables.

version: '3.8'
 
services:
  web:
    image: my-web-app:${VERSION}
    environment:
      VERSION: 1.0 # Or pass VERSION at runtime

Precedence and Overriding

Understand the order of precedence to avoid unexpected behavior:

1. Shell environment
2. docker-compose.yml (or .env if not explicitly overridden)
3. env_file
4. .env file in the project directory (loaded by default if not superseded)

Managing Secrets

Avoid storing sensitive information directly in your Compose file or .env files. Use Docker secrets for managing sensitive data securely.

version: '3.8'
 
services:
  db:
    image: postgres:latest
    secrets:
      - db_password
secrets:
  db_password:
    file: db_password.txt

Inspecting Environment Variables

Use docker-compose config to view the resolved environment variables for your services. This helps in debugging and verifying the correct values are being used.

Best Practices

• Use descriptive variable names.
• Keep .env files out of version control for security reasons.
• Leverage variable substitution for dynamic configuration.
• Utilize Docker secrets for sensitive data.
• Validate and test your configurations thoroughly.

By following these practices, you can effectively utilize environment variables within Docker Compose to streamline your development and deployment workflows.