Add support for -version argument

This will help determine the version of the binary particularly on Cloud
Native GitLab, where VERSION may not be shipped with the binaries.

Makefile

 .PHONY: validate verify verify_ruby verify_golang test test_ruby test_golang coverage coverage_golang setup _install build compile check clean
 
 GO_SOURCES := $(shell find . -name '*.go')
+VERSION_STRING := $(shell git describe --match v* 2>/dev/null || cat VERSION 2>/dev/null || echo unknown)
+BUILD_TIME := $(shell date -u +%Y%m%d.%H%M%S)
+GOBUILD_FLAGS := -ldflags "-X main.Version=$(VERSION_STRING) -X main.BuildTime=$(BUILD_TIME)"
 
 validate: verify test
 
@@ -37,7 +40,7 @@ _install:
 build: bin/gitlab-shell
 compile: bin/gitlab-shell
 bin/gitlab-shell: $(GO_SOURCES)
-	GOBIN="$(CURDIR)/bin" go install ./cmd/...
+	GOBIN="$(CURDIR)/bin" go install $(GOBUILD_FLAGS) ./cmd/...
 
 check:
 	bin/check

cmd/gitlab-shell/main.go

@@ -12,7 +12,21 @@ import (
 	"gitlab.com/gitlab-org/gitlab-shell/internal/logger"
 )
 
+var (
+	// Version is the current version of gitlab-shell
+	Version = "(unknown version)" // Set at build time in the Makefile
+	// BuildTime signifies the time the binary was build
+	BuildTime = "19700101.000000" // Set at build time in the Makefile
+)
+
 func main() {
+	// We can't use the flag library because gitlab-shell receives other arguments
+	// that confuse the parser.
+	if len(os.Args) == 2 && os.Args[1] == "-version" {
+		fmt.Printf("gitlab-shell %s-%s\n", Version, BuildTime)
+		os.Exit(0)
+	}
+
 	readWriter := &readwriter.ReadWriter{
 		Out:    os.Stdout,
 		In:     os.Stdin,