Stamp the git tag or short commit into the binary (closes #10)

rtnetmon had no version. main.Version is now set at link time and logged
in the first startup line. `make build` and `make dev` set it from
`git describe --tags --always` unless VERSION is given. The Dockerfile
takes it from the VERSION build argument when one is given, otherwise
from `git describe --tags --always` of the .git the build context now
carries, and fails the build if the context carries .git and no version
comes out. .dockerignore follows the canonical copy: .git is sent,
.git/config, which can hold a credential, is not.

Model: opus-5-5

cmd/rtnetmon/main.go

@@ -8,8 +8,12 @@ import (
 	"git.eeqj.de/sneak/rtnetmon/internal/cli"
 )
 
+// Version is the git tag or short commit, set at link time with -X by the
+// Makefile and the Dockerfile. Builds that do not set it report dev.
+var Version = "dev" //nolint:gochecknoglobals // set at link time with -X
+
 func main() {
-	err := cli.Execute()
+	err := cli.Execute(Version)
 	if err != nil {
 		log.Fatal(err)
 	}