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

Makefile

@@ -5,6 +5,8 @@
 # below are thin shims that call them.
 .DEFAULT_GOAL := check
 
+VERSION ?= $(shell git describe --tags --always)
+
 bootstrap:
 	@script/bootstrap
 
@@ -36,13 +38,14 @@ hooks:
 	@script/install-precommit
 
 build:
-	CGO_ENABLED=0 go build -trimpath -o bin/rtnetmon ./cmd/rtnetmon
+	CGO_ENABLED=0 go build -trimpath -ldflags "-X main.Version=$(VERSION)" \
+		-o bin/rtnetmon ./cmd/rtnetmon
 
 run: build
 	./bin/rtnetmon
 
 dev:
-	go run ./cmd/rtnetmon
+	go run -ldflags "-X main.Version=$(VERSION)" ./cmd/rtnetmon
 
 deps:
 	go mod download