build: add self-contained alpine-only Go image build

.project-docs/30-worklog/tasks/20260814-alpine-only-build-3b8e5a6d.md

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+# Task: Add self-contained alpine-only Go image build
+
+## Identity
+
+- Task ID: 20260814-alpine-only-build-3b8e5a6d
+- Mode: Feature
+- Branch: main
+- Worktree: /Users/brother7/Documents/AI/NianAIGC
+- Base commit: d82e7a104e84e0d6161adbda5339a6ad27088b91
+- Owner: dsh
+- Status: Ready for Integration
+
+## Scope
+
+- Provide a single-command docker build for the Go image that pulls only the alpine base image: the Go toolchain is installed from Alpine's package repositories (aliyun apk mirror) and module downloads use goproxy.cn, so no golang builder image and no external dockerfile frontend are needed.
+
+## Intent And Constraints
+
+- Runtime stage identical to the existing images (non-root uid/gid 10001, ca-certificates, tzdata).
+- Build-stage network dependencies are all China-accessible (aliyun apk mirror, goproxy.cn, sum.golang.google.cn).
+
+## Outcome
+
+- Added `backend/Dockerfile.alpine` with a self-contained two-stage build.
+- Documented it as the recommended build in `backend/README.md` and `docs/DEPLOYMENT.md`.
+
+## Verification
+
+- Dockerfile static review; the compile step is the same `CGO_ENABLED=0 go build` already verified locally (12.3 MB static ELF).
+- Actual image build must run on the CI machine: `docker build -f backend/Dockerfile.alpine -t REGISTRY/PROJECT/zhinian-go-api:TAG backend/`.
+
+## Follow-ups
+
+- Re-run the CI build with the new one-command path.
+
+## Promotion Candidates
+
+- None.