Keep a submodule's own .git/config out of the build context (closes #88)

A submodule that keeps its own `.git` directory, instead of one under `.git/modules/`, still shipped `sub/.git/config` into the build context, credential included. Both git patterns in the canonical `.dockerignore` now start with `**/`: `**/.git/config` and `**/.git/modules/**/config`.

A submodule whose name has a `config` segment (`config`, `deploy/config`, `config/lib`) still loses its whole git directory, because the pattern also matches that directory, and Go's version stamping then fails the build loudly. The file records this as a known gap with the way around it, `git submodule add --name`; closing it needs a wildcard re-include that makes every build walk excluded directories. `prompts/REPO_POLICIES.md` and both checklists say the same.

Model: opus-5-5

TODO.md

@@ -21,6 +21,15 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-04: The canonical `.dockerignore` now also keeps out the git `config`
+  of a submodule that keeps its own `.git` directory, which still reached the
+  image (issue 88): both git patterns now carry the `**/` prefix. A submodule
+  whose name has a `config` segment (`config`, `deploy/config`, `config/lib`)
+  still loses its whole git directory, so Go's version stamping fails the build;
+  the file records this as a `KNOWN GAP:` with the remedy,
+  `git submodule add --name`. Closing it would take a wildcard re-include, which
+  makes BuildKit walk every excluded directory, such as `node_modules`, on every
+  build. `REPO_POLICIES.md` and both checklists say so in the same words.
 - 2026-10-04: The note under the canonical Go `Dockerfile` example in
   `REPO_POLICIES.md` now installs lint-phase system libraries with `apt-get`
   under their Debian package names (issue 83). The `golangci/golangci-lint`