Use the git shortref as tag for Docker images. (#14901)

.circleci/config.yml

@@ -23,7 +23,7 @@ commands:
           name: Build docker image and push to repo
           command: |
             docker version
-            docker build -t app:build -f << parameters.dockerfile >> .
+            docker build -t app:build -f << parameters.dockerfile >> --label git.commit="$CIRCLE_SHA1" .
             docker tag app:build "${DOCKERHUB_REPO}":<< parameters.image_tag >>
             docker login -u "${DOCKERHUB_USER}" -p "${DOCKERHUB_PASS}"
             docker push "${DOCKERHUB_REPO}":<< parameters.image_tag >>