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/Dockerfile

@@ -25,12 +25,23 @@ RUN npm run build
 # Production stage
 FROM nginx:alpine
 
-# Copy custom nginx config
+# Install envsubst (gettext) so we can template nginx config at runtime
+RUN apk add --no-cache gettext
+
+# Copy nginx config template (will be processed at runtime)
+COPY frontend/nginx.conf.template /etc/nginx/nginx.conf.template
+# Also copy the original as fallback
 COPY frontend/nginx.conf /etc/nginx/nginx.conf
 
+# Copy entrypoint script
+COPY frontend/docker-entrypoint.sh /docker-entrypoint.sh
+RUN chmod +x /docker-entrypoint.sh
+
 # Copy built application from builder
 COPY --from=builder /app/frontend/dist /usr/share/nginx/html
 
 EXPOSE 80
 
+# Use custom entrypoint to process nginx config template
+ENTRYPOINT ["/docker-entrypoint.sh"]
 CMD ["nginx", "-g", "daemon off;"]