Use mirror.gcr.io for Docker base images to avoid Docker Hub rate limits

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -1,5 +1,5 @@
 # ---- Stage 1: Build frontend ----
-FROM node:22-alpine AS frontend-build
+FROM mirror.gcr.io/library/node:22-alpine AS frontend-build
 WORKDIR /app/frontend
 COPY frontend/package.json frontend/package-lock.json ./
 RUN npm ci
@@ -7,7 +7,7 @@ COPY frontend/ ./
 RUN npx vite build
 
 # ---- Stage 2: Build Go backend ----
-FROM golang:1.26-alpine AS backend-build
+FROM mirror.gcr.io/library/golang:1.26-alpine AS backend-build
 WORKDIR /app/backend
 COPY backend/go.mod backend/go.sum ./
 RUN go mod download
@@ -15,7 +15,7 @@ COPY backend/ ./
 RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /server ./cmd/server
 
 # ---- Stage 3: Runtime ----
-FROM alpine:3.21
+FROM mirror.gcr.io/library/alpine:3.21
 RUN apk add --no-cache ca-certificates tzdata
 WORKDIR /app