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

@@ -1,31 +1,32 @@
-# Lint stage
-# golangci/golangci-lint:v2.10.1-alpine, 2026-02-17
-FROM golangci/golangci-lint:v2.10.1-alpine@sha256:33bc6b6156d4c7da87175f187090019769903d04dd408833b83083ed214b0ddf AS lint
+# 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
 
-RUN apk add --no-cache make build-base vips-dev libheif-dev pkgconfig
+# 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 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
+# Build stage — tests and compilation
 # 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
 
+# 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 \