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.

backend/Dockerfile

@@ -1,7 +1,10 @@
+# CI can pass a Gitea-hosted mirror when Docker Hub TLS fails (see ci-resolve-node-image.ps1).
+ARG NODE_IMAGE=node:20-alpine
+
 # ============================================
 # STAGE 1: BUILDER STAGE
 # ============================================
-FROM node:20-alpine AS builder
+FROM ${NODE_IMAGE} AS builder
 
 WORKDIR /app
 
@@ -30,7 +33,7 @@ RUN npm prune --omit=dev
 # ============================================
 # STAGE 2: PRODUCTION STAGE
 # ============================================
-FROM node:20-alpine
+FROM ${NODE_IMAGE}
 
 RUN apk add --no-cache dumb-init