fix: syntax for docker username as env var

.github/workflows/publish.yml

@@ -30,7 +30,7 @@ jobs:
     - name: Log into Docker Hub
       uses: docker/login-action@v1 
       with:
-        username: secrets.DOCKERHUB_USERNAME
+        username: ${{ secrets.DOCKERHUB_USERNAME }}
         password: ${{ secrets.DOCKERHUB_ACCESS_TOKEN }}
 
     - name: Build and push Docker image