Debug, DockerHub username is not a secret

Author: SXT230118

.github/workflows/ci.yml

@@ -64,7 +64,7 @@ jobs:
       - name: Log in to DockerHub
         uses: docker/login-action@v3
         with:
-          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          username: ${{ vars.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
 
       # Generate image tags: