Fix nginx configuration syntax errors

- Move proxy_set_header directives inside location blocks
- Add DNS resolver for dynamic upstream resolution
- Improve fallback configuration in docker-compose
- Add config validation before starting nginx

This should resolve the nginx startup failures.

nginx-zero-downtime.conf

@@ -8,28 +8,15 @@ http {
         server portfolio-app-1:3000 max_fails=3 fail_timeout=30s;
         server portfolio-app-2:3000 max_fails=3 fail_timeout=30s;
     }
+    
+    # Resolver for dynamic upstream resolution
+    resolver 127.0.0.11 valid=10s;
 
     # Main server
     server {
         listen 80;
         server_name _;
 
-        # Proxy settings
-        proxy_set_header Host $host;
-        proxy_set_header X-Real-IP $remote_addr;
-        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
-        proxy_set_header X-Forwarded-Proto $scheme;
-
-        # Timeout settings
-        proxy_connect_timeout 5s;
-        proxy_send_timeout 60s;
-        proxy_read_timeout 60s;
-
-        # Buffer settings
-        proxy_buffering on;
-        proxy_buffer_size 4k;
-        proxy_buffers 8 4k;
-
         # Health check endpoint
         location /health {
             access_log off;
@@ -41,6 +28,22 @@ http {
         location / {
             proxy_pass http://portfolio_backend;
             
+            # Proxy settings
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+            proxy_set_header X-Forwarded-Proto $scheme;
+
+            # Timeout settings
+            proxy_connect_timeout 5s;
+            proxy_send_timeout 60s;
+            proxy_read_timeout 60s;
+
+            # Buffer settings
+            proxy_buffering on;
+            proxy_buffer_size 4k;
+            proxy_buffers 8 4k;
+            
             # Health check for upstream
             proxy_next_upstream error timeout invalid_header http_500 http_502 http_503 http_504;
             proxy_next_upstream_tries 2;
@@ -50,6 +53,13 @@ http {
         # Static files caching
         location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg|woff|woff2|ttf|eot)$ {
             proxy_pass http://portfolio_backend;
+            
+            # Proxy settings
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+            proxy_set_header X-Forwarded-Proto $scheme;
+            
             expires 1y;
             add_header Cache-Control "public, immutable";
         }