Split Dockerfile: pre-built golangci-lint stage for faster CI (#23)

## Summary

Splits the Dockerfile into a dedicated lint stage using the pre-built `golangci/golangci-lint:v2.10.1-alpine` Docker image, replacing the manual binary download with curl/sha256 verification.

## Changes

- **Lint stage** (`AS lint`): Uses `golangci/golangci-lint:v2.10.1-alpine` pinned by sha256. Runs `make fmt-check` + `make lint`. Includes CGO deps (`build-base`, `vips-dev`, `libheif-dev`, `pkgconfig`) needed for type-checking govips imports.
- **Build stage** (`AS builder`): Depends on lint stage via `COPY --from=lint /src/go.sum /dev/null`. Runs `make test` + builds the binary. Removes `curl` (no longer needed) and the manual golangci-lint download block.
- **Runtime stage**: Unchanged.

## Benefits

- Eliminates slow multi-arch binary download + sha256 verification step
- Lint and build stages can potentially run in parallel with BuildKit
- Better Docker layer caching — lint deps cached separately from build deps
- All images remain pinned by sha256 with version+date comments

## Verification

- `docker build .` passes: fmt-check ✅, lint (0 issues) ✅, all tests pass ✅, binary builds ✅

Closes [#18](#18)

<!-- session: agent:sdlc-manager:subagent:7aac9c54-81c8-4494-94ab-0843f97a1e62 -->

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #23
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

Dockerfile

@@ -1,7 +1,29 @@
+# Lint stage
+# golangci/golangci-lint:v2.10.1-alpine, 2026-02-17
+FROM golangci/golangci-lint:v2.10.1-alpine@sha256:33bc6b6156d4c7da87175f187090019769903d04dd408833b83083ed214b0ddf AS lint
+
+RUN apk add --no-cache make build-base vips-dev libheif-dev pkgconfig
+
+WORKDIR /src
+
+# Copy go mod files first for better layer caching
+COPY go.mod go.sum ./
+RUN go mod download
+
+# Copy source code
+COPY . .
+
+# Run formatting check and linter
+RUN make fmt-check
+RUN make lint
+
 # Build stage
 # golang:1.25.4-alpine, 2026-02-25
 FROM golang:1.25.4-alpine@sha256:d3f0cf7723f3429e3f9ed846243970b20a2de7bae6a5b66fc5914e228d831bbb AS builder
 
+# Depend on lint stage passing
+COPY --from=lint /src/go.sum /dev/null
+
 ARG VERSION=dev
 
 # Install build dependencies for CGO image libraries
@@ -9,15 +31,7 @@ RUN apk add --no-cache \
     build-base \
     vips-dev \
     libheif-dev \
-    pkgconfig \
-    curl
-
-# golangci-lint v2.10.1, 2026-02-25
-RUN curl -sSfL https://github.com/golangci/golangci-lint/releases/download/v2.10.1/golangci-lint-2.10.1-linux-amd64.tar.gz -o /tmp/golangci-lint.tar.gz && \
-    echo "dfa775874cf0561b404a02a8f4481fc69b28091da95aa697259820d429b09c99  /tmp/golangci-lint.tar.gz" | sha256sum -c - && \
-    tar -xzf /tmp/golangci-lint.tar.gz -C /tmp && \
-    mv /tmp/golangci-lint-2.10.1-linux-amd64/golangci-lint /usr/local/bin/ && \
-    rm -rf /tmp/golangci-lint*
+    pkgconfig
 
 WORKDIR /src
 
@@ -28,8 +42,8 @@ RUN GOTOOLCHAIN=auto go mod download
 # Copy source code
 COPY . .
 
-# Run all checks (fmt-check, lint, test)
-RUN make check
+# Run tests
+RUN make test
 
 # Build with CGO enabled
 RUN CGO_ENABLED=1 GOTOOLCHAIN=auto go build -ldflags "-X main.Version=${VERSION}" -o /pixad ./cmd/pixad