Say which Dockerfile stages run script/bootstrap (closes #90)

The bullet in `prompts/REPO_POLICIES.md` that requires a `Dockerfile` said every Dockerfile installs its prerequisites by running `script/bootstrap`, which the canonical Go `Dockerfile` in the same file never does. Reading the example as the deliberate one, the bullet now says the gate phases and the build stage start from their pinned base images and install what those images lack either inline, as the Go example does for `git`, or by running `script/bootstrap`, as this repository's own `Dockerfile` does for its yarn packages; the development environment stage of a non-server repo runs `script/bootstrap`. The new repo checklist item says the same.

The `COPY --from=` lines and the build stage's `apk add` line are unchanged.

Model: opus-5-5

TODO.md

@@ -21,6 +21,13 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-04: `REPO_POLICIES.md` now says which `Dockerfile` stages run
+  `script/bootstrap` (issue 90). The gate phases and the build stage start from
+  their pinned base images and install what those images lack either inline, as
+  the canonical Go `Dockerfile` does for `git`, or by running
+  `script/bootstrap`, as this repo's own `Dockerfile` does for its yarn
+  packages. The development environment stage, the final stage of a non-server
+  repo, runs `script/bootstrap`. The new repo checklist says the same.
 - 2026-10-04: Added a root `.gitattributes` that merges `TODO.md` with git's
   union merge (issue 98), so two branches that each add an entry at the top of
   Completed Steps merge without a conflict. Git now never reports a conflict in