Split Dockerfile into lint + build stages for faster CI feedback (#152)

## Summary

Splits the Dockerfile into separate lint and build stages to provide faster CI feedback on formatting and lint issues.

### Changes

**Dockerfile:**
- **Lint stage** (`golangci/golangci-lint:v2.10.1`, pinned by sha256): Runs `make fmt-check` and `make lint` using the official golangci-lint image which has the linter pre-installed. No more downloading golangci-lint on every build.
- **Build stage** (`golang:1.25-alpine`, pinned by sha256): Runs `make test` and `make build`. Same alpine image as before.
- **Runtime stage**: Unchanged.

**Makefile:**
- Added `fmt-check` target for standalone gofmt checking.
- Refactored `check` target to use `fmt-check`, `lint`, `test` as dependencies instead of inline commands. Still works identically for local use.

### Benefits
- Lint failures surface immediately without waiting for golangci-lint download
- Uses official pre-built golangci-lint image instead of manual binary download
- Cleaner separation of concerns between lint and build stages
- `make check` still runs everything sequentially for local development

closes #151

Co-authored-by: clawbot <clawbot@eeqj.de>
Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #152
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

Dockerfile

@@ -1,24 +1,6 @@
-# Build stage
+# Lint stage — fast feedback on formatting and lint issues
-# golang:1.25-alpine
+# golangci/golangci-lint:v2.10.1
-FROM golang@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4ced AS builder
+FROM golangci/golangci-lint@sha256:ea84d14c2fef724411be7dc45e09e6ef721d748315252b02df19a7e3113ee763 AS lint
-
-RUN apk add --no-cache git make gcc musl-dev
-
-# Install golangci-lint v2 (pre-built binary — go install fails in alpine due to missing linker)
-RUN set -e; \
-    GOLANGCI_VERSION="2.10.1"; \
-    case "$(uname -m)" in \
-        x86_64)  ARCH="amd64"; SHA256="dfa775874cf0561b404a02a8f4481fc69b28091da95aa697259820d429b09c99" ;; \
-        aarch64) ARCH="arm64"; SHA256="6652b42ae02915eb2f9cb2a2e0cac99514c8eded8388d88ae3e06e1a52c00de8" ;; \
-        *) echo "unsupported arch: $(uname -m)" >&2; exit 1 ;; \
-    esac; \
-    wget -q -O /tmp/golangci-lint.tar.gz \
-        "https://github.com/golangci/golangci-lint/releases/download/v${GOLANGCI_VERSION}/golangci-lint-${GOLANGCI_VERSION}-linux-${ARCH}.tar.gz"; \
-    echo "${SHA256}  /tmp/golangci-lint.tar.gz" | sha256sum -c -; \
-    tar -xzf /tmp/golangci-lint.tar.gz -C /usr/local/bin --strip-components=1 "golangci-lint-${GOLANGCI_VERSION}-linux-${ARCH}/golangci-lint"; \
-    rm /tmp/golangci-lint.tar.gz; \
-    golangci-lint version
-RUN go install golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0 # v0.42.0
 
 WORKDIR /src
 COPY go.mod go.sum ./
@@ -26,10 +8,22 @@ RUN go mod download
 
 COPY . .
 
-# Run all checks - build fails if any check fails
+RUN make fmt-check
-RUN make check
+RUN make lint
 
-# Build the binary
+# Build stage — tests and compilation
+# golang:1.25-alpine
+FROM golang@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4ced AS builder
+
+RUN apk add --no-cache git make gcc musl-dev
+
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+RUN make test
 RUN make build
 
 # Runtime stage

Makefile

@@ -1,4 +1,4 @@
-.PHONY: all build lint fmt test check clean
+.PHONY: all build lint fmt fmt-check test check clean
 
 BINARY := upaasd
 VERSION := $(shell git describe --tags --always --dirty 2>/dev/null || echo "dev")
@@ -18,20 +18,15 @@ fmt:
 	goimports -w .
 	npx prettier --write --tab-width 4 static/js/*.js
 
+fmt-check:
+	@test -z "$$(gofmt -l .)" || (echo "Files not formatted:" && gofmt -l . && exit 1)
+
 test:
 	go test -v -race -cover ./...
 
 # Check runs all validation without making changes
 # Used by CI and Docker build - fails if anything is wrong
-check:
+check: fmt-check lint test
-	@echo "==> Checking formatting..."
-	@test -z "$$(gofmt -l .)" || (echo "Files not formatted:" && gofmt -l . && exit 1)
-	@echo "==> Running linter..."
-	golangci-lint run --config .golangci.yml ./...
-	@echo "==> Running tests..."
-	go test -v -race ./...
-	@echo "==> Building..."
-	go build -ldflags "$(LDFLAGS)" -o /dev/null ./cmd/upaasd
 	@echo "==> All checks passed!"
 
 clean: