Go to transip.nl/cp/
log in and go to the API tab.
create a key-pair there
save the output there in key.private
If you cant to build this image yourself, simply run
docker build -t dyndns .
If you have build it yourself use the tag you used in previous step
docker run -it -v ./key.private:/key.private -e DOMAIN=example.com -e USERNAME=example_account -e SUBDOMAIN=dyndns xantios/dyndns
this wil setup dynamic DNS to your WAN ip on dyndns.example.com
assuming you own that domain in the account named example_account