chore: Update Dockerfile to use Node.js 20 and revise README for updated Node.js version

Dockerfile

@@ -1,5 +1,5 @@
 # Stage 1: Build React frontend
-FROM node:18-slim AS frontend-builder
+FROM node:20-slim AS frontend-builder
 WORKDIR /app/frontend
 
 # Copy package.json ONLY and install dependencies from scratch
@@ -12,7 +12,7 @@ COPY frontend/ .
 RUN npm run build
 
 # Stage 2: Setup Node.js backend and serve everything
-FROM node:18-slim
+FROM node:20-slim
 WORKDIR /app
 
 # Copy package.json ONLY and install production dependencies from scratch