fix docker commit tag [ci skip]
use the first 8 chars of the commit, not the last
# when building jupyter/repo2docker:master also push jupyter/repo2docker:abcd1234
-HASH_IMAGE="$DOCKER_REPO:${SOURCE_COMMIT: -8}"
+HASH_IMAGE="$DOCKER_REPO:${SOURCE_COMMIT:0:8}"
docker tag $DOCKER_REPO:$DOCKER_TAG $HASH_IMAGE
docker push $HASH_IMAGE
