fix commit hash truncation on docker images
truncate to 8 characters
# when building jupyter/repo2docker:master also push jupyter/repo2docker:abcd1234
-HASH_IMAGE="$DOCKER_REPO:${SOURCE_COMMIT:8}"
+HASH_IMAGE="$DOCKER_REPO:${SOURCE_COMMIT: -8}"
docker tag $DOCKER_REPO:$DOCKER_TAG $HASH_IMAGE
docker push $HASH_IMAGE
