split Dockerfile into lint + build stages for faster CI feedback

- Add dedicated lint stage using golangci/golangci-lint:v2.10.1 image
  (pinned by sha256 digest) for fast formatting and lint checks
- Keep build stage with golang:1.25-alpine for tests and compilation
- Remove manual golangci-lint download/install from builder stage
- Add fmt-check Makefile target for standalone format checking
- Refactor check target to use fmt-check, lint, test dependencies

The lint stage uses the official golangci-lint image which has the
linter pre-installed, eliminating the need to download it on every
build. Lint failures now surface immediately without waiting for
the download step.

closes #151

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:
-	@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
+check: fmt-check lint test
 	@echo "==> All checks passed!"
 
 clean: