You create a user on the central docker repository by running
docker login
If your username does not exist it will prompt you to also enter a password and your e-mail address. It will then automatically log you in.
In order to commit to the repository it is required to have committed your container to an image with your namespace.
# for example docker commit $CONTAINER_ID dhrp/kickassapp
docker commit <container_id> <your username>/<some_name>
In order to push an image to the repository you need to have committed your container to a named image (see above)
Now you can commit this image to the repository
# for example docker push dhrp/kickassapp
docker push <image-name>