build: make Go image build resilient to registry mirror failures

backend/README.md

@@ -37,6 +37,35 @@ npm run go:vet
 npm run go:build
 ```
 
+## Container images
+
+Standard multi-stage build (needs the `golang` builder image):
+
+```bash
+docker build -f backend/Dockerfile -t REGISTRY/PROJECT/zhinian-go-api:TAG backend/
+```
+
+Mirror-friendly build (override the base images when the registry accelerator
+cannot serve docker.io images):
+
+```bash
+docker build -f backend/Dockerfile \
+  --build-arg GOLANG_IMAGE=<mirror>/golang:1.21-alpine \
+  --build-arg RUNTIME_IMAGE=<mirror>/alpine:3.20 \
+  -t REGISTRY/PROJECT/zhinian-go-api:TAG backend/
+```
+
+Prebuilt-binary build (no `golang` image at all; compile the static binary on
+any Go 1.21 machine, then assemble from `alpine` only):
+
+```bash
+cd backend
+CGO_ENABLED=0 GOOS=linux GOARCH=amd64 \
+  go build -trimpath -ldflags="-s -w" -o zhinian-api.linux ./cmd/zhinian-api
+docker build -f backend/Dockerfile.runtime \
+  -t REGISTRY/PROJECT/zhinian-go-api:TAG backend/
+```
+
 The runner defaults to `CGO_ENABLED=0` for reproducible cross-platform builds.
 To exercise the local foundation manually without changing the existing Next
 server, use a different port: