refactor: use official golangci-lint image for lint stage

Restructure Dockerfile to match upaas/dnswatcher pattern:
- Separate lint stage using golangci/golangci-lint:v2.1.6 image
- Builder stage for tests and compilation (no lint dependency)
- Add fmt-check Makefile target
- Decouple test from lint in Makefile (lint runs in its own stage)
- Run gofmt on all files
- docker build verified passing locally

Dockerfile

@@ -1,57 +1,44 @@
-# Build stage
-FROM golang:1.24-alpine AS builder
+# Lint stage — fast feedback on formatting and lint issues
+# golangci/golangci-lint:v2.1.6
+FROM golangci/golangci-lint:v2.1.6 AS lint
 
-# Install build dependencies
-RUN apk add --no-cache \
-    gcc \
-    musl-dev \
-    make \
-    git \
-    gnupg
-
-# Set working directory
-WORKDIR /build
-
-# Copy go mod files
+WORKDIR /src
 COPY go.mod go.sum ./
-
-# Download dependencies
 RUN go mod download
 
-# Copy source code
 COPY . .
 
-# Install golangci-lint for checks (binary install to avoid Go version constraints)
-RUN wget -O- -nv https://raw.githubusercontent.com/golangci/golangci-lint/HEAD/install.sh | sh -s -- -b $(go env GOPATH)/bin v2.1.6
+RUN make fmt-check
+RUN make lint
 
-# Run all checks (lint, vet, test, build)
-RUN make check
+# Build stage — tests and compilation
+FROM golang:1.24-alpine AS builder
 
-# Build the final binary with version info
-RUN CGO_ENABLED=1 go build -v -o secret cmd/secret/main.go
+# Force BuildKit to run the lint stage
+COPY --from=lint /src/go.sum /dev/null
+
+RUN apk add --no-cache gcc musl-dev make git gnupg
+
+WORKDIR /build
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+RUN make test
+RUN CGO_ENABLED=1 go build -v -ldflags "-X 'git.eeqj.de/sneak/secret/internal/cli.Version=0.1.0' -X 'git.eeqj.de/sneak/secret/internal/cli.GitCommit=$(git rev-parse HEAD)'" -o secret cmd/secret/main.go
 
 # Runtime stage
 FROM alpine:latest
 
-# Install runtime dependencies
-RUN apk add --no-cache \
-    ca-certificates \
-    gnupg
+RUN apk add --no-cache ca-certificates gnupg
 
-# Create non-root user
 RUN adduser -D -s /bin/sh secret
 
-# Copy binary from builder
 COPY --from=builder /build/secret /usr/local/bin/secret
-
-# Ensure binary is executable
 RUN chmod +x /usr/local/bin/secret
 
-# Switch to non-root user
 USER secret
-
-# Set working directory
 WORKDIR /home/secret
 
-# Set entrypoint
-ENTRYPOINT ["secret"]
+ENTRYPOINT ["secret"]