fix: mirror node:20-alpine to Gitea when Docker Hub TLS times out

BuildKit was failing on registry-1.docker.io even when a local base
image existed. CI now prefers a Gitea-hosted NODE_IMAGE and retries
docker build.

frontend/Dockerfile

@@ -1,5 +1,8 @@
+# CI can pass a Gitea-hosted mirror when Docker Hub TLS fails (see ci-resolve-node-image.ps1).
+ARG NODE_IMAGE=node:20-alpine
+
 # Build stage — produces `.next/standalone` (see next.config.ts output: standalone)
-FROM node:20-alpine AS builder
+FROM ${NODE_IMAGE} AS builder
 
 WORKDIR /app
 
@@ -32,7 +35,7 @@ ENV NEXT_PUBLIC_SENTRY_ENVIRONMENT=${NEXT_PUBLIC_SENTRY_ENVIRONMENT}
 RUN npm run build
 
 # Production — minimal runtime using Next.js standalone bundle
-FROM node:20-alpine AS runner
+FROM ${NODE_IMAGE} AS runner
 
 RUN apk add --no-cache dumb-init