build: make Go image build resilient to registry mirror failures

.project-docs/30-worklog/tasks/20260814-go-image-build-fix-7d4b9e2c.md

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+# Task: Make Go image build resilient to mirror failures
+
+## Identity
+
+- Task ID: 20260814-go-image-build-fix-7d4b9e2c
+- Mode: Feature
+- Branch: main
+- Worktree: /Users/brother7/Documents/AI/NianAIGC
+- Base commit: 550edcdc059073d9bea96d4b577d4fafeaefdf74
+- Owner: dsh
+- Status: Ready for Integration
+
+## Scope
+
+- Fix the CI image build failure where the build machine's registry accelerator (mirror.aliyuncs.com) returned 500 on a `golang:1.21-alpine` blob: make the base images overridable and add a golang-image-free build path.
+
+## Intent And Constraints
+
+- Keep the standard multi-stage build as the default; no behavioral change for healthy registries.
+- The runtime image must stay non-root, root-filesystem-read-only compatible, and identical across both build paths.
+
+## Outcome
+
+- `backend/Dockerfile`: added `GOLANG_IMAGE` and `RUNTIME_IMAGE` ARGs so a working mirror can be supplied via `--build-arg` without editing the file.
+- Added `backend/Dockerfile.runtime`: alpine-only image that copies a prebuilt static `zhinian-api.linux` binary, for build environments that cannot pull the golang builder image at all.
+- Documented all three build paths (standard, mirror override, prebuilt binary) in `backend/README.md` and `docs/DEPLOYMENT.md`.
+
+## Verification
+
+- `CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -trimpath -ldflags="-s -w" -o zhinian-api.linux ./cmd/zhinian-api` succeeds on this machine: 12.3 MB statically linked stripped ELF x86-64, confirming the prebuilt-binary path is usable.
+- Docker image assembly itself must run on a machine with Docker; both Dockerfiles follow the same runtime stage as the already-merged image definition.
+
+## Follow-ups
+
+- On the CI machine, either switch the registry accelerator in `/etc/docker/daemon.json` or use the prebuilt-binary path.
+- Re-run the failing Gitea build to confirm.
+
+## Promotion Candidates
+
+- None.