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.

docker-compose.yml

@@ -35,6 +35,8 @@ services:
   frontend:
     image: ghcr.io/danielvolz/medassist-ng-frontend:latest
     container_name: medassist-ng-frontend
+    environment:
+      - BACKEND_URL=backend:3000
     ports:
       - "4174:8080"
     networks: