fix: hardcode Docker Hub URL (secrets not allowed in environment.url)

Author: sbryngelson

.github/workflows/docker.yml

@@ -139,7 +139,7 @@ jobs:
     needs: Container
     environment:
       name: containers
-      url: https://hub.docker.com/r/${{ secrets.DOCKERHUB_USERNAME }}/mfc
+      url: https://hub.docker.com/r/sbryngelson/mfc
     steps:
       - name: Login to Docker Hub
         uses: docker/login-action@v3