Based on Alpine Linux to be small
You can (and need) to connect your own /etc/ddclient to the container so it get your ddclient.conf. You can connect your own /var/cache/ddclient to keep your running environment between starts.
To start : docker run -d -v ~/etc/ddclient:/etc/ddclient -v ~/var/ddclient:/var/cache/ddclient --name ddclient --restart=always sbonnell/armhf-ddclient
To Build : docker build -t armhf-ddclient .