docker-compose.yml

version: '2'
services:
    wordpress:
        build:
            context: ./
            dockerfile: ./Dockerfile
        links:
            - db:mysql
            - mailhog
        ports:
            - 80:80
        domainname: foo.com
        hostname: foo
        volumes:
            - ./public:/var/www/html
        environment:
            WORDPRESS_DB_PASSWORD: example
        depends_on:
           - mailhog

    mailhog:
        image: mailhog/mailhog
        ports:
            - 1025:1025
            - 8025:8025

    db:
        image: mariadb
        environment:
            MYSQL_ROOT_PASSWORD: example

Dockerfile

FROM wordpress
RUN curl --location --output /usr/local/bin/mhsendmail https://github.com/mailhog/mhsendmail/releases/download/v0.2.0/mhsendmail_linux_amd64 && \
    chmod +x /usr/local/bin/mhsendmail

RUN echo 'sendmail_path="/usr/local/bin/mhsendmail --smtp-addr=mailhog:1025 --from=no-reply@docker.dev"' > /usr/local/etc/php/conf.d/mailhog.ini