fix: use env intermediary for secrets check in container-publish

GitHub Actions doesn't allow secrets in step-level if conditions.
Use env variable intermediary pattern instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/container-publish.yml

@@ -32,8 +32,10 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Log in to Docker Hub
-        if: ${{ secrets.DOCKERHUB_USERNAME != '' }}
+        if: env.DOCKERHUB_USERNAME != ''
         uses: docker/login-action@74a5d142397b4f367a81961eba4e8cd7edddf772 # v3.4.0
+        env:
+          DOCKERHUB_USERNAME: ${{ secrets.DOCKERHUB_USERNAME }}
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}