Let a plain docker build pass and stamp the git version (closes #57)

The size tests skip a case whose secret needs more locked memory than
RLIMIT_MEMLOCK allows: memguard panics otherwise, and a plain
`docker build .` runs under an 8 MiB limit. script/cibuild still runs
every case.

The build stage stamps the VERSION build argument, else
`git describe --tags --always`, and fails when .git is present but
yields no version. `make build` stamps `git describe` too instead of
the fixed 0.1.0. .dockerignore keeps .git/config out; script/docker is
now the canonical copy.

Model: opus-5-5

TODO.md

@@ -25,6 +25,14 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-10-02: A plain `docker build .` builds again: the size tests
+  skip a case that needs more locked memory than the limit allows,
+  and run every case under `script/cibuild`. The image stamps the
+  `VERSION` build argument, else `git describe --tags --always`, into
+  `Version`, and fails if `.git` is present but yields no version;
+  `make build` stamps `git describe` too, not a fixed `0.1.0`.
+  `.dockerignore` keeps `.git/config` out; `script/docker` is the
+  canonical copy.
 - 2026-08-07: Updated golangci-lint to v2.12.2 with the canonical
   `.golangci.yml` (all linters enabled minus the standard disable
   list, `lll` 88, tests linted); bumped the `Dockerfile` lint-stage