fix: add nginx resolver and proxy_ssl_server_name for external HTTPS proxy

nginx requires an explicit resolver when proxy_pass uses a variable.
Also enable proxy_ssl_server_name so TLS SNI works correctly when
proxying to the HTTPS backend on HuggingFace Spaces.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -23,9 +23,12 @@ RUN printf 'server {\n\
     index index.html;\n\
 \n\
     location /api/ {\n\
-        proxy_pass ${VITE_API_BASE_URL}/api/;\n\
-        proxy_set_header Host $http_host;\n\
+        resolver 8.8.8.8 valid=30s;\n\
+        set $backend ${VITE_API_BASE_URL};\n\
+        proxy_pass $backend/api/;\n\
+        proxy_set_header Host $proxy_host;\n\
         proxy_set_header X-Real-IP $remote_addr;\n\
+        proxy_ssl_server_name on;\n\
     }\n\
 \n\
     location / {\n\