move version fetch into target

Makefile

@@ -3,25 +3,29 @@ CWD := $(shell pwd)
 NODE_VERSION ?= 12.20.1-r0
 LESS_VERSION ?=
 
-VERSION_URL ?= https://www.npmjs.com/package/less
-VERSION_PATTERN ?= '(?<="latest":")[^"]+(?=")'
-ifndef (LESS_VERSION)
-	LESS_VERSION = $(shell curl -s $(VERSION_URL) | grep -Po $(VERSION_PATTERN))
-endif
-
 IMAGE_NAME ?= sndsgd/less
-IMAGE := $(IMAGE_NAME):$(LESS_VERSION)
 
 .PHONY: help
 help:
 	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) \
 	| awk 'BEGIN {FS = ":.*?## "}; {printf "\033[33m%s\033[0m~%s\n", $$1, $$2}' \
 	| column -s "~" -t
 
+VERSION_URL ?= https://www.npmjs.com/package/less
+VERSION_PATTERN ?= '(?<="latest":")[^"]+(?=")'
+.PHONY: ensure-version
+ensure-version:
+ifeq ($(LESS_VERSION),)
+	$(info fetching latest version...)
+	@$(eval LESS_VERSION = $(shell curl -s $(VERSION_URL) | grep -Po $(VERSION_PATTERN)))
+endif
+	@$(eval IMAGE := $(IMAGE_NAME):$(LESS_VERSION))
+
 IMAGE_ARGS ?= --quiet
 .PHONY: image
 image: ## Build the docker image
-	@echo "building image for less v$(LESS_VERSION)..."
+image: ensure-version
+	$(info building image for less v$(LESS_VERSION)...)
 	@docker build \
 	  $(IMAGE_ARGS) \
 		--build-arg NODE_VERSION=$(NODE_VERSION) \
@@ -34,14 +38,15 @@ image: ## Build the docker image
 test: ## Test the docker image
 test: image
 	@make --no-print-directory execute-test \
+		LESS_VERSION=$(LESS_VERSION) \
 		TEST_NAME=one
 
 TEST_ARGS ?=
 TEST_INPUT ?= source.less
 TEST_NAME ?=
 .PHONY: execute-test
-execute-test:
-	@echo "testing '$(TEST_NAME)'..."
+execute-test: ensure-version
+	$(info testing '$(TEST_NAME)'...)
 	@docker run --rm -t \
 		-v $(CWD):$(CWD) \
 		-w $(CWD) $(IMAGE) \