fix: use Node 20 for Next.js 16 compatibility

Dockerfile

@@ -1,15 +1,15 @@
-FROM node:18-alpine AS deps
+FROM node:20-alpine AS deps
 WORKDIR /app
 COPY package*.json ./
 RUN npm install
 
-FROM node:18-alpine AS builder
+FROM node:20-alpine AS builder
 WORKDIR /app
 COPY --from=deps /app/node_modules ./node_modules
 COPY . .
 RUN npm run build
 
-FROM node:18-alpine AS runner
+FROM node:20-alpine AS runner
 WORKDIR /app
 ENV NODE_ENV=production
 ENV PORT=7860