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>

backend/Dockerfile

@@ -1,5 +1,6 @@
 # ---- Stage 1: Build ----
-FROM node:24-alpine AS builder
+ARG BASE_IMAGE=node:24-alpine
+FROM ${BASE_IMAGE} AS builder
 
 WORKDIR /app
 
@@ -10,7 +11,7 @@ COPY . .
 RUN npm run build
 
 # ---- Stage 2: Production ----
-FROM node:24-alpine AS production
+FROM ${BASE_IMAGE} AS production
 
 RUN apk add --no-cache dumb-init curl bash \
  && curl -fsSL https://get.helm.sh/helm-v3.15.4-linux-amd64.tar.gz | tar xz -C /tmp \