To enable DKIM signature, you must have created your mail accounts.
Once its done, just run:

    docker run --rm \
      -v "$(pwd)/config":/tmp/docker-mailserver \
      -ti tvial/docker-mailserver:v2 generate-dkim-config

Now the keys are generated, you can configure your DNS server by just pasting the content of `config/opedkim/keys/domain.tld/mail.txt` in your `domain.tld.hosts` zone.