Add a Makefile with a few targets. The default is 'build'. The build

target calls the new version-at-commit.sh script which will
automatically populate the version variable inside the Headscale binary.

Once we start tagging releases on the git tree, that will come in handy.

The Makefile also has a 'test' target (does nothing yet, no tests yet)
and a 'dev' target, which runs linters, tests, and finally builds.

cmd/headscale/headscale.go

@@ -17,7 +17,7 @@ import (
 	"tailscale.com/tailcfg"
 )
 
-const version = "0.1"
+var version = "dev"
 
 var versionCmd = &cobra.Command{
 	Use:   "version",