chore: Add makefile and use in gitlab-ci.yml

Gives an easy possibility to execute the same commands as in the pipeline.

.gitlab-ci.yml

@@ -7,15 +7,14 @@ stages:
 fmt:
   stage: validate
   script:
-    - gofmt -l -s -e .
-    - exit $(gofmt -l -s -e . | wc -l)
+    - make check-fmt
 
 vet:
   stage: validate
   script:
-    - go vet -composites=false ./...
+    - make vet
 
 test:
   stage: validate
   script:
-    - go test -race -v ./...
+    - make test

Makefile

@@ -0,0 +1,47 @@
+# kernel-style V=1 build verbosity
+ifeq ("$(origin V)", "command line")
+	BUILD_VERBOSE = $(V)
+endif
+
+ifeq ($(BUILD_VERBOSE),1)
+	Q =
+else
+	Q = @
+endif
+
+define go_get
+	$(Q)command -v $(1) > /dev/null || GO111MODULE=off go get $(2)
+endef
+
+export CGO_ENABLED := 1
+
+.DEFAULT_GOAL := help
+
+.PHONY: validate
+validate: check-fmt test vet ## Validates the go code format, runs tests and executes vet.
+
+.PHONY: test
+test: ## Run tests
+	$(Q)echo "running tests..."
+	$(Q)go test -race -v ./...
+
+.PHONY: vet
+vet: ## Run go vet
+	$(Q)echo "running go vet..."
+	$(Q)go vet -composites=false ./...
+
+.PHONY: check-fmt
+check-fmt: ## Check go format
+	$(Q)echo "checking format..."
+	@gofmt_out=$$(gofmt -d -e . 2>&1) && [ -z "$${gofmt_out}" ] || (echo "$${gofmt_out}" 1>&2; exit 1)
+
+.PHONY: fmt
+fmt: ## Formats the go code
+	$(Q)echo "formatting go code..."
+	$(Q)set -e
+	$(call go_get,goimports,golang.org/x/tools/cmd/goimports)
+	$(Q)goimports -w .
+
+.PHONY: help
+help: ## Shows this help
+	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | awk 'BEGIN {FS = ":.*?## "}; {printf "\033[36m%-30s\033[0m %s\n", $$1, $$2}'