fix: use dynamic BACKEND_URL for nginx reverse proxy (#118)

Fixes #96

- nginx.conf converted to template processed by envsubst at container start
- BACKEND_URL env var (default: backend:3000) replaces hardcoded container name
- Docker DNS resolver used for dynamic upstream resolution
- Dockerfile copies nginx.conf as template to /etc/nginx/templates/

This prevents frontend breakage when users customize container names
in their docker-compose.yml.

frontend/nginx.conf

@@ -20,7 +20,14 @@ server {
     }
 
     location /api/ {
-        proxy_pass http://medassist-ng-backend:3000/;
+        # Use variable for runtime DNS resolution (nginx resolves at startup by default)
+        # Docker embedded DNS (127.0.0.11) with 10s cache
+        resolver 127.0.0.11 valid=10s ipv6=off;
+        set $backend_upstream ${BACKEND_URL};
+
+        # Strip /api prefix (nginx doesn't auto-rewrite when using variables in proxy_pass)
+        rewrite ^/api/(.*)$ /$1 break;
+        proxy_pass http://$backend_upstream;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;