Build ckan-base image (2.9)
cd /path/to/ckan-docker/ckan/ckan-base
docker build -t ckan/ckan-base:testing-only.2.9 -f 2.9/Dockerfile .
 
Login to Dockerhub
docker login --username=kowhai
(will then ask for a password)

Push image to (ckan) Dockerhub
docker push ckan/ckan-base:testing-only.2.9

Do the same with 2.8 and 2.7