ci: pull node base image from Harbor instead of docker.io

Kaniko builds failed with context deadline exceeded pulling node:24-alpine
from index.docker.io through the egress proxy. Seed node:24-alpine into
abrban/ and pass BASE_IMAGE build-arg so builds use the internal registry.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/Dockerfile

@@ -1,5 +1,6 @@
 # ---- Stage 1: Dependencies ----
-FROM node:24-alpine AS deps
+ARG BASE_IMAGE=node:24-alpine
+FROM ${BASE_IMAGE} AS deps
 
 WORKDIR /app
 COPY package.json package-lock.json* ./
@@ -8,7 +9,7 @@ RUN sed -i 's|https://registry.npmmirror.com|https://registry.npmjs.org|g' packa
  && npm ci
 
 # ---- Stage 2: Build ----
-FROM node:24-alpine AS builder
+FROM ${BASE_IMAGE} AS builder
 
 WORKDIR /app
 COPY --from=deps /app/node_modules ./node_modules
@@ -20,7 +21,7 @@ ENV NEXT_TELEMETRY_DISABLED=1
 RUN npm run build
 
 # ---- Stage 3: Production ----
-FROM node:24-alpine AS production
+FROM ${BASE_IMAGE} AS production
 
 RUN apk add --no-cache dumb-init