Rails development environment with Docker (step 2 - postgres)

docker-compose.yml

web:
        build: ./
        command: bundle exec rails s -p 3000 -b '0.0.0.0'
        volumes:
                - .:/app
        ports:
                - "3000:3000"
        links:
                - db
db:
        image: postgres:9.4
        expose:
                - "5432"