add docker hub generalization

docker_hub.gitlab-ci.yml

+include:
+- local: images.gitlab-ci.yml
+
+variables:
+  DOCKER_LOGIN_CMD: docker login --username $DOCKER_HUB_USERNAME --password $DOCKER_HUB_ACCESS_TOKEN
+  DOCKER_LOGOUT_CMD: docker logout
+  IMAGE_PREFIX: $DOCKER_HUB_USERNAME