makefile example best practices / opinionated from https://tech.davis-hansson.com/p/make/

Makefile

SHELL := bash
.ONESHELL:
.SHELLFLAGS := -eu -o pipefail -c
.DELETE_ON_ERROR:
MAKEFLAGS += --warn-undefined-variables
MAKEFLAGS += --no-builtin-rules

ifeq ($(origin .RECIPEPREFIX), undefined)
  $(error This Make does not support .RECIPEPREFIX. Please use GNU Make 4.0 or later)
endif
.RECIPEPREFIX = >

.PHONY: help
.DEFAULT: help
help: ## show the usage
	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | sort | awk -F ':.*?## ' 'NF==2 {printf "%-35s %s\n", $$1, $$2}'

.PHONY: build
build: out/image-id ## build it

.PHONY: clean
clean: ## Clean up the output directories; since all the sentinel files go under tmp, this will cause everything to get rebuilt
> rm -rf tmp
> rm -rf out

tmp/.tests-passed.sentinel: $(shell find src -type f) ## Tests - re-ran if any file under src has been changed since tmp/.tests-passed.sentinel was last touched
> mkdir -p $(@D)
> node run test
> touch $@ 

tmp/.packed.sentinel: tmp/.tests-passed.sentinel ## Webpack - re-built if the tests have been rebuilt (and so, by proxy, whenever the source files have changed)
> mkdir -p $(@D)
> webpack ..
> touch $@

out/image-id: tmp/.packed.sentinel ## Docker image - re-built if the webpack output has been rebuilt
> mkdir -p $(@D)
> image_id="example.com/my-app:$$(pwgen -1)"
> docker build --tag="$${image_id}
> echo "$${image_id}" > out/image-id