docs: update README for environment variable configuration and clarify usage in Next.js

docker: modify Dockerfile to set NODE_ENV from environment variable for flexibility

dockerfile

@@ -25,7 +25,7 @@ FROM node:22-alpine AS runner
 WORKDIR /app
 
 # Set environment variables
-ENV NODE_ENV production
+ENV NODE_ENV $NODE_ENV
 ENV NEXT_TELEMETRY_DISABLED 1 
 
 # Add a non-root user for security