How to have a makefile only rebuild a docker image when needed

Makefile

DOCKER_TAG = ...

.PHONY: build-image touch.build-image
build-image: touch.build-image .build-image

touch.build-image:
	$(eval timestamp=$(shell docker inspect -f '{{ range $$i, $$e := split .Metadata.LastTagTime "T" }}{{if eq $$i 0}}{{range $$j, $$v := split $$e "-"}}{{$$v}}{{end}}{{else}}{{$$f := printf "%.8s" $$e}}{{range $$j, $$g := split $$f ":"}}{{if lt $$j 2}}{{$$g}}{{else}}.{{$$g}}{{end}}{{end}}{{end}}{{end}}' $(DOCKER_TAG) 2>/dev/null ))
	@if [ "A$(timestamp)A" = "AA" ] ; then rm -f .build-image ; else touch -t $(timestamp) .build-image ; fi

.build-image: Dockerfile rootfs/*
	docker build --tag $(DOCKER_TAG):latest .
	@touch .build-image