feat(gh-actions): skip docker.io/voxpupuli login on forks

Author: saimonn

.github/workflows/build_container.yml

@@ -99,6 +99,7 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Log in to the docker.io registry
+        if: secrets.DOCKERHUB_BOT_ADMIN_TOKEN != ''
         uses: docker/login-action@v3
         with:
           registry: docker.io