add dockerhub post_push

to publish git-tagged images

hooks/README.md

@@ -0,0 +1,3 @@
+# Docker build hooks
+
+These define our [custom hooks for docker automated builds](https://docs.docker.com/docker-hub/builds/advanced/#custom-build-phase-hooks)

hooks/post_push

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+# when building jupyter/repo2docker:master also push jupyter/repo2docker:abcd1234
+
+HASH_IMAGE="$DOCKER_REPO:${SOURCE_COMMIT:8}"
+docker tag $DOCKER_REPO:$DOCKER_TAG $HASH_IMAGE
+docker push $HASH_IMAGE