make: add clean target (#344)

Signed-off-by: xhe <xw897002528@gmail.com>

Makefile

@@ -27,7 +27,7 @@ endif
 IMAGE_TAG ?= latest
 EXECUTABLE_TARGETS := $(patsubst cmd/%,cmd_%,$(wildcard cmd/*))
 
-.PHONY: cmd_% test lint docker docker-release golangci-lint gocovmerge
+.PHONY: cmd_% test lint docker docker-release golangci-lint gocovmerge clean
 
 default: cmd
 
@@ -79,6 +79,9 @@ test: gocovmerge
 	rm -f .cover.*
 	go tool cover -html=.cover -o .cover.html
 
+clean:
+	rm -rf bin dist grafonnet-lib
+
 docker:
 	docker build -t "$(DOCKERPREFIX)tiproxy:$(IMAGE_TAG)" --build-arg "GOPROXY=$(shell go env GOPROXY)" --build-arg "VERSION=$(VERSION)" --build-arg "COMMIT=$(COMMIT)" -f docker/Dockerfile .