Stamp the git commit into images built from the Dockerfile (closes #236)

.dockerignore left out .git, so `make build` in the image found no git
metadata and stamped `dev`. It now sends .git, and no longer leaves out
tracked files (LICENSE, README.md, ...), which git would see as deleted
and mark the version -dirty. The footer and /health already read the
same version. The startup log line that reports it, the logger's
Identify(), was never called; main now calls it. The README says to
build from a git clone.
Side effect: image layers after `COPY . .` now rebuild whenever `.git` changes.
Disclosure: merged after a rebase that changed only TODO.md; the second review gated this same tree on this base.

Model: opus-5-5

cmd/upaasd/main.go

@@ -52,6 +52,8 @@ func main() {
 			handlers.New,
 			server.New,
 		),
-		fx.Invoke(func(*server.Server) {}),
+		fx.Invoke(func(log *logger.Logger, _ *server.Server) {
+			log.Identify()
+		}),
 	).Run()
 }