build: drop external Dockerfile frontend to avoid mirror pulls

.project-docs/30-worklog/tasks/20260814-remove-syntax-frontend-1f6c9d3b.md

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+# Task: Drop external Dockerfile frontend to avoid mirror pulls
+
+## Identity
+
+- Task ID: 20260814-remove-syntax-frontend-1f6c9d3b
+- Mode: Feature
+- Branch: main
+- Worktree: /Users/brother7/Documents/AI/NianAIGC
+- Base commit: c7d2cb23d0e6866d2cd49e1d1032d90e8cef7863
+- Owner: dsh
+- Status: Ready for Integration
+
+## Scope
+
+- Fix the CI image build failure where BuildKit tried to pull the `docker/dockerfile:1` frontend image through the broken `registry.docker-cn.com` mirror and timed out before any project step ran.
+
+## Intent And Constraints
+
+- The three Dockerfiles only use built-in frontend features (multi-stage COPY, ARG-before-FROM, RUN/ENV), so removing the `# syntax=` pin loses nothing and skips the external frontend download entirely.
+
+## Outcome
+
+- Removed `# syntax=docker/dockerfile:1` from `Dockerfile`, `backend/Dockerfile`, and `backend/Dockerfile.runtime`.
+
+## Verification
+
+- Static check: no `syntax=docker` directive remains; every Dockerfile still contains a `FROM` line.
+- Actual image assembly must run on the build machine; the change removes the failing pull step (the frontend image) from the build graph.
+
+## Follow-ups
+
+- On the CI machine, replace the dead `registry.docker-cn.com` mirror in `/etc/docker/daemon.json` with a live one (e.g. `docker.m.daocloud.io`) and restart Docker; or build via `backend/Dockerfile.runtime` with a prebuilt binary to avoid the golang image as well.
+
+## Promotion Candidates
+
+- None.