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

frontend/docker-entrypoint.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+set -e
+
+# Set default backend URL if not provided
+export BACKEND_URL="${BACKEND_URL:-backend:8000}"
+
+echo "Configuring nginx with BACKEND_URL: ${BACKEND_URL}"
+
+# Substitute environment variables in nginx config template
+# Only substitute BACKEND_URL, preserve nginx variables like $http_upgrade
+envsubst '${BACKEND_URL}' < /etc/nginx/nginx.conf.template > /etc/nginx/nginx.conf
+
+# Execute the main command (nginx)
+exec "$@"
+