fix: add COPY --from=lint to builder stage to force lint execution (#154)

BuildKit skips unreferenced stages silently. The lint stage (added in PR [#152](#152)) was never referenced by the builder stage via `COPY --from`, so it was being skipped entirely during `docker build .`. Linting was not actually running in CI.

This adds `COPY --from=lint /src/go.sum /dev/null` to the builder stage, creating a stage dependency that forces the lint stage to complete before the build proceeds.

Verified: `docker build .` now runs the lint stage (fmt-check + lint) and passes.

closes #153

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #154
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

Dockerfile

@@ -15,6 +15,9 @@ RUN make lint
 # golang:1.25-alpine
 FROM golang@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4ced AS builder
 
+# Force BuildKit to run the lint stage by creating a stage dependency
+COPY --from=lint /src/go.sum /dev/null
+
 RUN apk add --no-cache git make gcc musl-dev
 
 WORKDIR /src