Add Makefile help

Makefile

@@ -2,8 +2,14 @@ VERSION = 0.11.0
 TAG = $(VERSION)
 PREFIX = nginx/nginx-prometheus-exporter
 
+.DEFAULT_GOAL:=help
+
+.PHONY: help
+help: Makefile ## Display this help
+	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | sort | awk 'BEGIN {FS = ":.*?## "; printf "Usage:\n\n    make \033[36m<target>\033[0m [VARIABLE=value...]\n\nTargets:\n\n"}; {printf "    \033[36m%-30s\033[0m %s\n", $$1, $$2}'
+
 .PHONY: nginx-prometheus-exporter
-nginx-prometheus-exporter:
+nginx-prometheus-exporter: ## Build nginx-prometheus-exporter binary
 	CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X github.com/prometheus/common/version.Version=$(VERSION)" -o nginx-prometheus-exporter
 
 .PHONY: build-goreleaser
@@ -12,26 +18,26 @@ build-goreleaser: ## Build all binaries using GoReleaser
 	GOPATH=$(shell go env GOPATH) goreleaser build --clean --snapshot
 
 .PHONY: lint
-lint:
+lint: ## Run linter
 	docker run --pull always --rm -v $(shell pwd):/nginx-prometheus-exporter -w /nginx-prometheus-exporter -v $(shell go env GOCACHE):/cache/go -e GOCACHE=/cache/go -e GOLANGCI_LINT_CACHE=/cache/go -v $(shell go env GOPATH)/pkg:/go/pkg golangci/golangci-lint:latest golangci-lint --color always run
 
 .PHONY: test
-test:
+test: ## Run tests
 	go test ./... -race -shuffle=on
 
 .PHONY: container
-container:
+container: ## Build container image
 	docker build --build-arg VERSION=$(VERSION) --target container -f build/Dockerfile -t $(PREFIX):$(TAG) .
 
 .PHONY: push
-push: container
+push: container ## Push container image
 	docker push $(PREFIX):$(TAG)
 
 .PHONY: deps
-deps:
+deps: ## Add missing and remove unused modules, verify deps and download them to local cache
 	@go mod tidy && go mod verify && go mod download
 
 .PHONY: clean
-clean:
+clean: ## Clean up
 	-rm -r dist
 	-rm nginx-prometheus-exporter