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

README.md

@@ -226,6 +226,10 @@ This recipe serves plain HTTP, so `UPAAS_PLAINTEXT_HTTP=true` is required for
 setup and every other form to pass the CSRF origin check. Behind a
 TLS-terminating reverse proxy, drop that line.
 
+The image shows the commit it was built from (the `git describe` output) in the
+page footer, the startup log and `/health`. Build it from a git clone: without
+the `.git` directory it shows `dev`.
+
 ### Deploying with Docker Compose
 
 [`docker-compose.yml`](docker-compose.yml) builds the image from this repo and