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

prompts/NEW_REPO_CHECKLIST.md

@@ -124,8 +124,10 @@ are thin shims calling them. Model scripts:
 - [ ] `script/bootstrap` / `make bootstrap` — installs all dependencies,
       idempotently, assuming nothing (pkg manager detection nix/apt/brew/apk;
       node used if present, else pinned version via nvm from a hash-verified
-      archive; pinned yarn via corepack); Dockerfile runs it instead of inline
-      installs
+      archive; pinned yarn via corepack); a non-server repo's development
+      environment stage runs it instead of inline installs; a gate phase or the
+      build stage installs what its base image lacks either inline or by running
+      it
 - [ ] `script/setup` / `make setup` — readies a fresh clone: runs `bootstrap`,
       then `install-precommit`, plus repo-specific init
 - [ ] `script/test` / `make test` — `docker build --no-cache --target test .`,