Stamp the tag or short commit in a plain docker build (closes #112)

.dockerignore now sends .git but not .git/config, which can hold a
credential and which git describe does not need. The build stage stamps
main.Gitrev from the VERSION build argument when one is given, otherwise
from git describe --tags --always, and fails if .git is present and no
version comes out. git there trusts /src whoever owns it, since a
context sent as a tar archive keeps its files' owners. script/docker is
replaced by the canonical copy, which passes VERSION; bin/gitrev.sh uses
--tags too, so every entrypoint stamps the same value for a clean
commit.

Model: opus-5-5

TODO.md

@@ -24,6 +24,12 @@ only thing left of the `chore/align-repo-policies` branch is the list below.
 
 # Completed Steps
 
+- 2026-10-02: a plain `docker build .` of a clone now stamps the tag or short
+  commit into `mfer version` instead of nothing: `.dockerignore` sends `.git`
+  (not `.git/config`), and the build stage takes the `VERSION` build argument,
+  otherwise `git describe --tags --always`, failing if `.git` is present and no
+  version comes out. `script/docker` is the canonical copy, which passes
+  `VERSION`; `bin/gitrev.sh` uses `--tags` too (#112)
 - 2026-09-21: validate manifest entry paths on deserialize so untrusted `.mf`
   files cannot make `Checker` stat or read outside `basePath` (#61)
 - 2026-09-21: rewrote `script/test` to the canonical pattern (30s timeout,