refactor: update Docker configuration & docs

- Modified docker-compose.yml to clarify that the server runs as a non-root user.
- Updated Dockerfile to use ARG for VITE_SERVER_URL, allowing build-time overrides.
- Replaced inline Nginx configuration with a separate nginx.conf file for better maintainability.
- Adjusted documentation to reflect changes in Docker setup and troubleshooting steps.

apps/ui/nginx.conf

@@ -0,0 +1,10 @@
+server {
+    listen 80;
+    server_name localhost;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+}