fix: update Docker Hub login to use secrets for credentials

Author: kadykov

.github/workflows/ci.yml

@@ -106,7 +106,7 @@ jobs:
       - name: Log in to Docker Hub
         uses: docker/login-action@v3
         with:
-          username: ${{ vars.DOCKERHUB_USERNAME }}
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
 
       - name: Semantic Release