Say that a build from a linked worktree needs the version passed in (closes #111)

A plain `docker build .` from a checkout whose `.git` is a file (a linked worktree, or a repository checked out as a submodule) stops at the version check of the canonical `Dockerfile` example: that file points to a git directory outside the build context, so `git describe` prints nothing. The check is right to refuse an empty version; what was missing is what to do. `prompts/REPO_POLICIES.md` and both checklists now say such a build is given its version with `--build-arg VERSION=...`, as `script/docker` and `script/cibuild` already do. The check itself is unchanged.

Model: opus-5-5

TODO.md

@@ -21,6 +21,14 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-06: `REPO_POLICIES.md` and both checklists now say that a checkout
+  whose `.git` is a file, a linked worktree or a repository checked out as a
+  submodule, is the exception to a plain `docker build .` succeeding (issue
+  111): that file points to a git directory outside the build context, so the
+  build cannot read the version and the version check in the canonical
+  `Dockerfile` stops it. Such a build is given its version with
+  `--build-arg VERSION=...`, as `script/docker` and `script/cibuild` already do.
+  The check itself is unchanged.
 - 2026-10-06: The canonical `.gitea/workflows/check.yml` now has a `concurrency`
   block, so a new push cancels the older run on the same branch and no other,
   and its checkout step sets `persist-credentials: false`, so the job's token is