chore: prefer git tag for docker image tag (#768)

We should prefer to use a tag for images over the default branch when
applicable.

Also, since we're filtering by the default branch in the workflow, we
don't need to validate the branch name in the job.

.circleci/config.yml

@@ -382,13 +382,19 @@ jobs:
       - run:
           name: Restore Docker image cache
           command: docker load -i /tmp/cache/docker.tar
+      # This is the easiest way to tag multiple images using different
+      # conditions for the GAR_TAG variable in the smallest amount of code.
+      #
+      # You can find other jobs and commands you can use with this orb that
+      # include tagging here:
+      # https://circleci.com/developer/orbs/orb/circleci/gcp-gcr
       - run:
           name: Tag image
           command: |
-            if [ "${CIRCLE_BRANCH}" == "master" ]; then
-              echo 'export GAR_TAG=master' >> $BASH_ENV
-            elif [ ! -z "${CIRCLE_TAG}" ]; then
-              echo "export GAR_TAG=$CIRCLE_TAG" >> $BASH_ENV
+            if [ ! -z "${CIRCLE_TAG}" ]; then
+              echo "export GAR_TAG=${CIRCLE_TAG}" >> $BASH_ENV
+            else
+              echo "export GAR_TAG=${CIRCLE_BRANCH}" >> $BASH_ENV
             fi
               echo "export GAR_IMAGE=\"<<parameters.registry-url>>/${GCP_GAR_PROJECT_ID}/${GCP_GAR_REPO}/<<parameters.image>>\"" >> $BASH_ENV
               source $BASH_ENV