docker-compose up -d

web:
        build: .
        restart: always
        command: ['tini', '--', 'rails', 's']
        environment:
            RAILS_ENV: production
            HOST: example.com
            EMAIL: admin@example.com
        links:
                - db:mongo
                - exim4:exim4.docker # <-- Add link
        ports:
                - 3000:3000
        volumes:
                - .:/usr/src/app
db:
        image: mongo
        restart: always
exim4: # <-------------------------------- Add new container
        image: exim4
        restart: always
        ports:
            - 25:25
        environment:
            EMAIL_USER: user@example.com
            EMAIL_PASSWORD: abcdabcdabcdabcd