Read Netbird management URL from secret

Switch the deploy job's Netbird Connect step to read management-url from
the NETBIRD_URL secret instead of a hardcoded value, so the URL can be
managed in the Gitea UI alongside the setup key.

.github/workflows/build-docker-images.yml

@@ -58,7 +58,7 @@ jobs:
         with:
           setup-key: ${{ secrets.NETBIRD_SETUP_KEY }}
           hostname: 'gitea-actions-blog'
-          management-url: 'https://netbird.ahmza.com'
+          management-url: ${{ secrets.NETBIRD_URL }}
 
       - name: Deploy to RPIC-Cluster
         uses: appleboy/ssh-action@v1