impr(ci): retry container image pushing and send slack messages for failures (#11416)

## Problem
We've seen quite a few CI failures related to pushes to docker hub
failing with weird error messages that indicate maybe docker hub is just
not reliable.

## Summary of changes
Retry container image pushing up to 10 times, and send a slack message
if we had to retry, regardless of the job succeeding or not.

.github/workflows/_push-to-container-registry.yml

@@ -104,6 +104,18 @@ jobs:
           password: ${{ secrets.NEON_DOCKERHUB_PASSWORD }}
 
       - name: Copy docker images to target registries
+        id: push
         run: python3 .github/scripts/push_with_image_map.py
         env:
           IMAGE_MAP: ${{ inputs.image-map }}
+
+      - name: Notify Slack if container image pushing fails
+        if: steps.push.outputs.slack_notify == 'true' || failure()
+        uses: slackapi/slack-github-action@485a9d42d3a73031f12ec201c457e2162c45d02d # v2.0.0
+        with:
+          method: chat.postMessage
+          token: ${{ secrets.SLACK_BOT_TOKEN }}
+          payload: |
+            channel: ${{ vars.SLACK_ON_CALL_DEVPROD_STREAM }}
+            text: |
+              Pushing container images failed in <${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}|GitHub Run>