docker: report the real version in the image (closes #109)

The Dockerfile builder stage takes ARG VERSION (default `dev`) and passes
it to `make build` on the command line, which overrides the Makefile's
`git describe` default. script/docker computes the version from
`git describe` on the host and passes it as --build-arg VERSION, because
.dockerignore leaves .git out of the build context and `git describe`
inside the build only ever produced `dev`. A build that passes no
argument, such as script/cibuild, still reports `dev`.

`logger.Identify`, which logs `starting` with the version, was never
called; `main` now calls it first, so the version is in the startup log.

Model: opus-5-5

cmd/dnswatcher/main.go

@@ -63,6 +63,7 @@ func main() {
 				return n
 			},
 		),
+		fx.Invoke(func(l *logger.Logger) { l.Identify() }),
 		fx.Invoke(func(*server.Server, *watcher.Watcher) {}),
 	).Run()
 }