Add backend URL configuration for frontend and update nginx setup

- Added BACKEND_URL environment variable to docker-compose for frontend service.
- Introduced a new entrypoint script to configure nginx with the BACKEND_URL at runtime.
- Created a template for nginx configuration to handle API and WebSocket requests dynamically.
- Updated README with instructions for configuring reverse proxy setups.

Fixes #12

docker-compose.yml

@@ -32,6 +32,8 @@ services:
     container_name: excalidash-frontend
     ports:
       - "6767:80"
+    environment:
+      - BACKEND_URL=backend:8000
     depends_on:
       - backend
     networks: