ci: add failure notifications for v3 build

Author: darora

.github/workflows/qemu-image-build.yml

@@ -144,6 +144,16 @@ jobs:
           docker tag "postgres:$IMAGE_TAG" "$REGISTRY/$REPOSITORY:$IMAGE_TAG"
           docker push "$REGISTRY/$REPOSITORY:$IMAGE_TAG"
 
+      - name: Slack Notification on Failure
+        if: ${{ failure() }}
+        uses: rtCamp/action-slack-notify@v2
+        env:
+          SLACK_WEBHOOK: ${{ secrets.SLACK_NOTIFICATIONS_WEBHOOK }}
+          SLACK_USERNAME: 'gha-failures-notifier'
+          SLACK_COLOR: 'danger'
+          SLACK_MESSAGE: 'Building Postgres QEMU artifact failed'
+          SLACK_FOOTER: ''
+b
       - name: Cleanup resources after build
         if: ${{ always() }}
         run: |