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

internal/cli/export_test.go

@@ -3,4 +3,4 @@ package cli
 import "github.com/spf13/cobra"
 
 // NewRootCmd exposes newRootCmd for external tests.
-func NewRootCmd() *cobra.Command { return newRootCmd() }
+func NewRootCmd() *cobra.Command { return newRootCmd("dev") }