fix: remove invalid secrets check in GitHub Actions workflow

- Fix syntax error with secrets.SLACK_WEBHOOK check
- Use continue-on-error instead for optional Slack notifications
- Workflow will now run successfully even without SLACK_WEBHOOK secret

Author: jeremi

.github/workflows/docker-build.yml

@@ -234,8 +234,9 @@ jobs:
 
     steps:
       - name: Notify Slack
-        if: secrets.SLACK_WEBHOOK != ''
         uses: 8398a7/action-slack@v3
+        if: always()
+        continue-on-error: true
         with:
           status: ${{ job.status }}
           text: |