Replace docker-build script with multi-stage build

Makefile

@@ -9,8 +9,6 @@ DOCKER_REPO=quay.io/coreos/dex
 DOCKER_IMAGE=$(DOCKER_REPO):$(VERSION)
 
 $( shell mkdir -p bin )
-$( shell mkdir -p _output/images )
-$( shell mkdir -p _output/bin )
 
 user=$(shell id -u -n)
 group=$(shell id -g -n)
@@ -58,12 +56,8 @@ lint:
       golint -set_exit_status $$package $$i || exit 1; \
 	done
 
-_output/bin/dex:
-	@./scripts/docker-build
-	@sudo chown $(user):$(group) _output/bin/dex
-
 .PHONY: docker-image
-docker-image: clean-release _output/bin/dex
+docker-image:
 	@sudo docker build -t $(DOCKER_IMAGE) .
 
 .PHONY: proto
@@ -85,13 +79,9 @@ bin/protoc-gen-go:
 check-go-version:
 	@./scripts/check-go-version
 
-clean: clean-release
+clean:
 	@rm -rf bin/
 
-.PHONY: clean-release
-clean-release:
-	@rm -rf _output/
-
 testall: testrace vet fmt lint
 
 FORCE: