impr(ci): send clearer notifications to slack when retrying container image pushes (#11447)

## Problem
We've started sending slack notifications for failed container image
pushes that are being retried. There are more messages coming in than
expected, so clicking through the link to see what image failed is
happening more often than we hoped.

## Summary of changes
- Make slack notifications clearer, including whether the job succeeded
and what retries have happened.
- Log failures/retries in step more clearly, so that you can easily see
when something fails.

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

@@ -110,12 +110,19 @@ jobs:
           IMAGE_MAP: ${{ inputs.image-map }}
 
       - name: Notify Slack if container image pushing fails
-        if: steps.push.outputs.slack_notify == 'true' || failure()
+        if: steps.push.outputs.push_failures || 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>
+            text: >
+              *Container image pushing ${{
+                steps.push.outcome == 'failure' && 'failed completely' || 'succeeded with some retries'
+              }}* in
+              <${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}|GitHub Run>
+
+              ${{ steps.push.outputs.push_failures && format(
+                '*Failed targets:*\n• {0}', join(fromJson(steps.push.outputs.push_failures), '\n• ')
+              ) || '' }}