Add Makefile documentation (#25)

Add help texts to Makefile targets:

  all             Alias for `build`
  build           Build runtime Docker image
  build.develop   Build develop container image
  develop         Run interactive shell inside developer container
  fmt             Auto-format source code and report code-style violations (lint)
  help            Print Makefile documentation
  run             Run make target inside developer container (e.g. `make run fmt`)
  test            Run tests

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Signed-off-by: ddelange <[email protected]>

Makefile

@@ -23,32 +23,45 @@ ifeq (run,$(firstword $(MAKECMDGOALS)))
 endif
 
 .PHONY: all
+## Alias for `build`
 all: build
 
 .PHONY: build
+## Build runtime Docker image
 build:
 	docker build -t ${IMG_NAME}:latest --target runtime .
 
 .PHONY: build.develop
+## Build develop container image
 build.develop:
 	docker build -t ${IMG_NAME}-develop:latest --target develop .
 
 .PHONY: develop
+## Run interactive shell inside developer container
 develop: build.develop
 	./scripts/develop.sh
 
 .PHONY: run
+## Run make target inside developer container (e.g. `make run fmt`)
 run: build.develop
 	./scripts/develop.sh make $(RUN_ARGS)
 
 .PHONY: fmt
+## Auto-format source code and report code-style violations (lint)
 fmt:
 	./scripts/fmt.sh
 
 .PHONY: test
+## Run tests
 test:
 	go test -coverprofile cover.out `go list ./...`
 
+.DEFAULT_GOAL := help
+.PHONY: help
+## Print Makefile documentation
+help:
+	@perl -0 -nle 'printf("\033[36m  %-15s\033[0m %s\n", "$$2", "$$1") while m/^##\s*([^\r\n]+)\n^([\w.-]+):[^=]/gm' $(MAKEFILE_LIST) | sort
+
 # Override targets if they are included in RUN_ARGs so it doesn't run them twice
 # otherwise 'make run fmt' would be equivalent to calling './scripts/develop.sh make fmt'
 # followed by 'make fmt'