Fix Node.js version requirement

- Update Dockerfile to use Node.js 20 (required for Next.js 16)
- Next.js 16 requires Node.js >=20.9.0, was using 18.20.8

Dockerfile

@@ -1,5 +1,5 @@
-# Use the official Node.js 18 Alpine image as the base
+# Use the official Node.js 20 Alpine image as the base
-FROM node:18-alpine AS base
+FROM node:20-alpine AS base
 
 # Install dependencies and build in the same stage for simplicity
 FROM base AS builder