fix(ci): replace python3 with cat heredoc; sync local registry IPs

python3 is not in PATH inside dotnet/sdk:10.0 container — replace the
"Write NuGet config" step with a cat heredoc which works in any container.

Also syncs GitHub with the Gitea-side changes:
  - All images pulled from local Nexus mirrors (no internet round-trip)
      171.22.25.73:5000 → docker-hub-proxy (node, postgres, redis)
      171.22.25.73:5002 → mcr-proxy        (dotnet/sdk)
  - npm steps already on npm-group (Liara + Runflare fallback)
  - docker-compose.mirror.yml: expose port 5002 for mcr-proxy

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.mirror.yml

@@ -30,7 +30,8 @@ services:
       - nexus-data:/nexus-data
     ports:
       - "8081:8081"   # Web UI + NuGet + npm REST API
-      - "5000:5000"   # Docker Hub pull-through proxy (httpPort configured in docker-hub-proxy repo)
+      - "5000:5000"   # Docker Hub proxy  (docker-hub-proxy repo  httpPort: 5000)
+      - "5002:5002"   # MCR proxy         (mcr-proxy repo          httpPort: 5002)
     healthcheck:
       test: ["CMD", "curl", "-sf", "http://localhost:8081/service/rest/v1/status"]
       interval: 30s