feat: split Dockerfile into dedicated lint stage

Use pre-built golangci/golangci-lint:v2.1.6 image for fast lint feedback
instead of installing golangci-lint from source on every build.

- Lint stage: runs fmt-check and lint using pre-built image
- Build stage: runs tests and compiles binaries
- COPY --from=lint forces BuildKit to execute the lint stage
- All images pinned by sha256 digest
- Runtime stage unchanged

Dockerfile

@@ -1,18 +1,28 @@
+# Lint stage — fast feedback on formatting and lint issues
+# golangci/golangci-lint:v2.1.6
+FROM golangci/golangci-lint@sha256:568ee1c1c53493575fa9494e280e579ac9ca865787bafe4df3023ae59ecf299b AS lint
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN make fmt-check
+RUN make lint
+
+# Build stage — tests and compilation
 # golang:1.24-alpine, 2026-02-26
 FROM golang@sha256:8bee1901f1e530bfb4a7850aa7a479d17ae3a18beb6e09064ed54cfd245b7191 AS builder
 WORKDIR /src
 RUN apk add --no-cache git build-base make
 
-# golangci-lint v2.1.6 (eabc2638a66d), 2026-02-26
-RUN CGO_ENABLED=0 go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@eabc2638a66daf5bb6c6fb052a32fa3ef7b6600d
+# Force BuildKit to run the lint stage by creating a stage dependency
+COPY --from=lint /src/go.sum /dev/null
 
 COPY go.mod go.sum ./
 RUN go mod download
 
 COPY . .
 
-# Run all checks — build fails if branch is not green
-RUN make check
+RUN make test
 
 # Build static binaries (no cgo needed at runtime — modernc.org/sqlite is pure Go)
 ARG VERSION=dev