Switch back to building Docker images on a schedule.

.circleci/config.yml

@@ -71,13 +71,13 @@ workflows:
     # Build the Docker images used by the ci jobs.  This makes the ci jobs
     # faster and takes various spurious failures out of the critical path.
     triggers:
-      # # Build once a day
-      # - schedule:
-      #     cron: "0 0 * * *"
-      #     filters:
-      #       branches:
-      #         only:
-      #           - "master"
+      # Build once a day
+      - schedule:
+          cron: "0 0 * * *"
+          filters:
+            branches:
+              only:
+                - "master"
 
     jobs:
       # Every job that pushes a Docker image from Docker Hub needs to provide