Derive the image version from git; send .git without its config (closes #69, closes #71)

The canonical documents told every repo to exclude .git from the build
context, default ARG VERSION to dev and never run git describe in a
build stage, so an image built from a clone with no build argument
reported dev. .dockerignore now sends .git but keeps out .git/config,
which can hold a credential. The Dockerfile example installs git, takes
the VERSION build argument when one is given and otherwise
git describe --tags --always, and fails when .git exists but the version
is empty, dev or unknown. The policy and both checklists state the rule
in the same words, including that a plain docker build . with no build
arguments must succeed.

Model: opus-5-5

.gitignore

@@ -20,26 +20,8 @@ Thumbs.db
 # Node
 node_modules/
 
-# Secrets. Unanchored like every entry above, so each matches at every
+# Environment / secrets
-# depth. Matching is case-sensitive on Linux, so names use character
+.env
-# ranges rather than a lowercase form that misses `Server.Key`.
+.env.*
-
+*.pem
-# Environment files. `*.env` covers bare `.env` and the `prod.env`
+*.key
-# convention. Only the templates `example.env` and `sample.env` are
-# re-included below. A repository that commits any other template adds
-# its own negation after these lines, for example `!.env.example`.
-*.[eE][nN][vV]
-.[eE][nN][vV].*
-.[eE][nN][vV][rR][cC]
-!example.env
-!sample.env
-
-# Private keys and the bundles carrying them.
-*.[pP][eE][mM]
-*.[kK][eE][yY]
-*.[pP]12
-*.[pP][fF][xX]
-[iI][dD]_[rR][sS][aA]
-[iI][dD]_[dD][sS][aA]
-[iI][dD]_[eE][cC][dD][sS][aA]
-[iI][dD]_[eE][dD]25519

TODO.md

@@ -21,11 +21,6 @@ fmt-check, and commit.
 
 # Completed Steps
 
-- 2026-10-03: Brought the canonical `.gitignore` level with `.dockerignore` on
-  secrets (issue 38): it now also ignores `prod.env`-style `*.env` files,
-  `.envrc`, `*.p12`, `*.pfx` and the extensionless SSH private keys, written to
-  `.gitignore`'s own rules (no `**/` prefix) and case-folded with character
-  ranges. `example.env` and `sample.env` stay trackable through negations.
 - 2026-10-02: The image version now comes from git inside the build (issues 69
   and 71), superseding the 2026-09-08 entry that excluded `.git`. The canonical
   `.dockerignore` sends `.git` but keeps out `.git/config`, which can hold a