docker-ddclient is a unofficial docker image for ddclient (sf, github). docker-ddclient is based on on Alpine linux for simplicity and small footprint.

The image itself is very simple exposing no network ports and only a single volume (/etc/ddclient). One way to start a container could be:

docker run \
    --detach \
    --name ddclient \
    --restart always \
    --volume /etc/ddclient:/etc/ddclient:rw \
    gcscaglia/docker-ddclient:latest

With the above comand, ddclient will run in daemon mode, start with the system and read it's configurations from /etc/ddclient/ddclient.conf in the host's filesystem.

For info on how to craft the ddclient.conf file, please refer to the documentation of ddclient itself.

By default containers don't have access to the host's network interfaces, instead accessing the network through the docker bridge.

If your ddclient.conf determines your IP using web method, this difference in networking is irrelevant. If, instead, it uses the interface (if) method the easier solution is to change the container networking to host mode. i.e:

docker run \
    --detach \
    --name ddclient \
    --restart always \
    --volume /etc/ddclient:/etc/ddclient:rw \
    --net host \
    gcscaglia/docker-ddclient:latest

With host networking, all host interfaces are exposed directly to your container and can be used as-is.