chore: add Makefile with check gate, golangci-lint config, dockerignore

- make check runs lint + vet + test-short; lint is pinned to the go.mod
  toolchain so it works when the host Go is newer than golangci-lint's
- gofumpt main.go and patch.go; drop unused gatewayGroup/gatewayKind vars

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

main.go

@@ -4,8 +4,8 @@ import (
 	"flag"
 	"os"
 
-	gozap "go.uber.org/zap"
 	cmv1 "github.com/cert-manager/cert-manager/pkg/apis/certmanager/v1"
+	gozap "go.uber.org/zap"
 	"k8s.io/apimachinery/pkg/runtime"
 	utilruntime "k8s.io/apimachinery/pkg/util/runtime"
 	clientgoscheme "k8s.io/client-go/kubernetes/scheme"