Report the version: docker build . stamps the git tag or short commit (closes #7)

attrsum --version now prints the git tag or short commit. make build
stamps it with -X from git describe, and the Dockerfile takes the VERSION
build argument when given, otherwise git describe --tags --always on the
.git in the build context, failing if .git is present and no version
comes out. A new .dockerignore, the canonical one, keeps .git/config out
of the context, and also this repo's host-built /attrsum. CI checks out
full history so it sees the 1.0.0 tag and stamps what a full clone does.
script/docker is replaced with the canonical copy.

Model: opus-5-5

TODO.md

@@ -26,6 +26,11 @@ $(HOME)/Documents/_SYSADMIN/cyberdyne path.
 
 # Completed Steps
 
+* 2026-10-02: `attrsum --version` reports the git tag or short commit,
+    stamped by `make build` and by a plain `docker build .` of a clone;
+    `.dockerignore` sends `.git` without `.git/config` and keeps a
+    host-built `attrsum` out; CI checks out full history so it stamps
+    the same version
 * 2026-02-02: correctness pass: track actual bytes read instead of
     stale file size, atomic failure tracking in ProcessCheck, detect
     file modification during checksum (TOCTOU), propagate countFiles
@@ -39,8 +44,6 @@ $(HOME)/Documents/_SYSADMIN/cyberdyne path.
 
 # Future Steps
 
-* Add Dockerfile and .dockerignore that run make check, images pinned
-    by sha256, plus a Makefile docker target
 * Add .gitea/workflows/check.yml
 * Restructure README.md into the standard sections: Description,
     Getting Started, Rationale, Design, TODO, License, Author (Getting