feat: split Dockerfile into dedicated lint and build stages

- Add dedicated lint stage using pre-built golangci-lint v2.10.1 image
- Move fmt-check and lint from build stage to lint stage for faster feedback
- Remove manual golangci-lint binary download (now handled by lint image)
- Remove curl from build stage dependencies (no longer needed)
- Add COPY --from=lint dependency to force BuildKit to run lint stage
- Build stage now runs only tests and compilation

closes #20

Dockerfile

@@ -1,23 +1,38 @@
-# Build stage
+# Lint stage — fast feedback on formatting and lint issues
+# golangci/golangci-lint:v2.10.1, 2026-03-01
+FROM golangci/golangci-lint@sha256:ea84d14c2fef724411be7dc45e09e6ef721d748315252b02df19a7e3113ee763 AS lint
+
+# Install CGO dependencies needed for static analysis of vips/libheif code
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libvips-dev \
+    libheif-dev \
+    pkg-config \
+    && rm -rf /var/lib/apt/lists/*
+
+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:1.25.4-alpine, 2026-02-25
 FROM golang:1.25.4-alpine@sha256:d3f0cf7723f3429e3f9ed846243970b20a2de7bae6a5b66fc5914e228d831bbb AS builder
 
 ARG VERSION=dev
 
+# Force BuildKit to run the lint stage by creating a stage dependency
+COPY --from=lint /src/go.sum /dev/null
+
 # Install build dependencies for CGO image libraries
 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 +43,8 @@ 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