fix: nginx serves index.html with no-cache headers

Vite hashes all JS/CSS assets, so those can be cached for 30 days.
But index.html must never be cached — it references the current hashes.
Caching index.html meant users ran old JS after deploys.

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

Dockerfile

@@ -19,6 +19,10 @@ RUN echo 'server { \
     listen 3000; \
     root /usr/share/nginx/html; \
     index index.html; \
+    location = /index.html { \
+    expires -1; \
+    add_header Cache-Control "no-store, no-cache, must-revalidate"; \
+    } \
     location / { \
     try_files $uri $uri/ /index.html; \
     expires 30d; \