Notify on slack when the dogfood_merge fails (#4834)

.github/workflows/dogfood.yml

@@ -34,3 +34,12 @@ jobs:
     # Use the output from the `dogfood` step
     - name: Get the name of the dogfood branch and its HEAD SHA1
       run: echo "The dogfood branch was `${{ steps.dogfood.outputs.dogfood-branch }}` and its HEAD SHA1 was `${{ steps.dogfood.outputs.sha1 }}`"
+    # Notify on Slack
+    - name: Notify failures on Slack
+      if: failure()
+      uses: slackapi/slack-github-action@70cd7be8e40a46e8b0eced40b0de447bdb42f68e # v1.26.0
+      env:
+        SLACK_BOT_TOKEN: ${{ fromJSON(steps.secrets.outputs.vault).SLACK_BOT_TOKEN }}
+      with:
+        channel-id: squad-jvm-notifs
+        slack-message: "Dogfood build for `${{ steps.dogfood.outputs.sha1 }}`: *failed*, see the logs at https://github.com/SonarSource/sonar-java/actions/workflows/dogfood.yml"