postgres:
    image: postgres:9.6
redis:
    image: redis:4.0
app:
    image: freecodecamp/mail-for-good:latest
    env_file: .env
    environment:
        - PSQL_USERNAME=postgres
        - PSQL_DATABASE=postgres
        - PSQL_HOST=postgres
        - REDIS_HOST=redis
        - VIRTUAL_HOST=app
        - VIRTUAL_PORT=8080
    links:
        - redis
        - postgres
    ports:
      - "8080:8080"

postgres:
    image: postgres:9.6.3
redis:
    image: redis:4.0
app:
    image: freecodecamp/mail-for-good:latest
    env_file: .env
    environment:
        - PSQL_USERNAME=postgres
        - PSQL_DATABASE=postgres
        - PSQL_HOST=postgres
        - REDIS_HOST=redis
        - VIRTUAL_HOST=app
        - VIRTUAL_PORT=8080
    links:
        - redis
        - postgres
    ports:
      - "8080:8080"

Set postgres version to 9.6.3
Set postgres version to 9.6.3
