chore: upgrade node:21-alpine to node:22-alpine (Active LTS)

Node 21 was a non-LTS release, EOL June 2024. Node 22 is the current
Active LTS, supported through April 2027.

Dockerfile

@@ -1,6 +1,6 @@
 # Stage 1: Build the application
-# Using node:21-alpine for a lightweight, secure build environment.
-FROM node:21-alpine AS builder
+# node:22-alpine — Node 22 is the current Active LTS (supported through April 2027).
+FROM node:22-alpine AS builder
 WORKDIR /app
 
 # Install dependencies first for better Docker layer caching.
@@ -30,7 +30,7 @@ RUN if [ "$BUILD_MODE" = "prod" ] || [ "$BUILD_MODE" = "production" ]; then \
     fi
 
 # Stage 2: Final runtime image
-FROM node:21-alpine AS deploy-node
+FROM node:22-alpine AS deploy-node
 WORKDIR /app
 
 # Copy only the built files and necessary scripts from the builder stage.