OPENDKIM on Alpine Linux 3.6

For those looking for a minimal image running OpenDKIM service.

Build the image

sudo docker build -t opendkim .

Run the container

sudo /usr/bin/docker run -d -v /etc/opendkim:/etc/opendkim:rw -p 127.0.0.1:12301:12301 --name=dkim1 opendkim

*This binds the docker port 12301 only on the local interface of the docker host for security. You may eventually instead want to give the container a fix IP and reference it from Postfix.*

Use the systemd file

cp systemd/docker-container@dkim1.service to /etc/systemd/system/
systemctl enable docker-container@dkim1.service
systemctl start docker-container@dkim1.service

Update Postfix

Add these lines to /etc/postfix/main.cf:

milter_protocol = 2
milter_default_action = accept
smtpd_milters = inet:DKIM_CONTAINER_IP:12301
non_smtpd_milters = inet:DKIM_CONTAINER_IP:12301

Howto configure OpenDKIM?

Have a look here: https://www.devops.zone/mailserver/how-to-configure-dkim-for-your-domain-with-postfix/