Fix Docker Hub login condition in CI workflow

Replace invalid secrets check in if-condition with a
repository variable (vars.DOCKERHUB_ENABLED) to avoid
workflow file parsing errors.

.github/workflows/docker-publish.yml

@@ -37,7 +37,7 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Log in to Docker Hub
-        if: github.event_name != 'pull_request' && secrets.DOCKERHUB_USERNAME != ''
+        if: github.event_name != 'pull_request' && vars.DOCKERHUB_ENABLED == 'true'
         uses: docker/login-action@v3
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}