build: Dockerfile.backend multistage lint stage (closes #17)

Dockerfile.backend did not follow the Go multistage lint-stage pattern
REPO_POLICIES.md mandates, and dragged the whole git history into the
build context to resolve a version string.

- Add an `AS lint` stage on the hash-pinned golangci/golangci-lint
  image (v2.7.2, the same golangci-lint commit main already pins), which
  ships Go, gofmt, make and the linter, so nothing is installed in it.
  It runs `make fmt-check` then `make lint`.
- Add `COPY --from=lint /src/go.sum /dev/null` to the build stage so
  BuildKit cannot run the two stages in parallel and let a lint failure
  through.
- Stop compiling golangci-lint from source in the build stage.
- Drop `COPY .git /repo/.git`; the version now comes from
  `ARG VERSION=dev`, passed to the build via `make build VERSION=...`.
- Drop gcc and musl-dev, and the corresponding
  `-linkmode external -extldflags -static` in backend/Makefile. The
  build is now `CGO_ENABLED=0 go build -trimpath` with
  `-ldflags "-s -w -X main.Version=... -X main.Buildarch=..."`, which is
  static without a C toolchain.
- backend/Makefile's VERSION is now overridable and degrades to `dev`
  when git or .git is unavailable instead of emitting a git error and
  building an empty version string.
- Every FROM stays pinned by @sha256 with a version and date comment.

Runtime stage, exposed port and entrypoint are unchanged.

Dockerfile.backend

@@ -1,25 +1,50 @@
+# Lint stage — fast feedback on formatting and lint issues. The
+# golangci/golangci-lint image ships Go, gofmt, make and the linter, so
+# nothing is installed here.
+# golangci/golangci-lint:v2.7.2 (2026-08-09)
+FROM golangci/golangci-lint@sha256:5d6d5c70a61f1356adfd9dd6316ce286799fefc9d743421356ff1b00842368ba AS lint
+
+WORKDIR /src
+COPY backend/go.mod backend/go.sum ./
+RUN go mod download
+COPY backend/ .
+RUN make fmt-check
+RUN make lint
+
+# Build stage
 # golang:1.25-alpine (2026-02-27)
 FROM golang:1.25-alpine@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4ced AS builder
 
-RUN apk add --no-cache git make gcc musl-dev
+RUN apk add --no-cache make
 
-# golangci-lint v2.7.2 (2026-02-27)
-RUN CGO_ENABLED=0 go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@9f61b0f53f80672872fced07b6874397c3ed197b
+WORKDIR /src
+
+# Force BuildKit to run the lint stage before proceeding. BuildKit runs
+# stages in parallel by default; without this no-op copy a lint failure
+# would not gate compilation.
+COPY --from=lint /src/go.sum /dev/null
 
-WORKDIR /repo/backend
 COPY backend/go.mod backend/go.sum ./
 RUN go mod download
-COPY .git /repo/.git
 COPY backend/ .
 
-RUN make check
-RUN make build
+RUN make test
 
+# The build is driven through the Makefile so there is exactly one
+# definition of the build command; it expands to
+# CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.Version=... -X main.Buildarch=..."
+# VERSION is handed over in the environment rather than as a make
+# variable so it still reaches the build if the target is ever turned
+# into a shim around a script.
+ARG VERSION=dev
+RUN VERSION="${VERSION}" make build
+
+# Runtime stage
 # alpine:3.23 (2026-02-27)
 FROM alpine:3.23@sha256:25109184c71bdad752c8312a8623239686a9a2071e8825f20acb8f2198c3f659
 
 RUN apk add --no-cache ca-certificates
-COPY --from=builder /repo/backend/netwatch-server /usr/local/bin/netwatch-server
+COPY --from=builder /src/netwatch-server /usr/local/bin/netwatch-server
 
 EXPOSE 8080
 ENTRYPOINT ["netwatch-server"]