Install lint-phase libraries with apt-get, not apk (closes #83)

A key point under the canonical Go `Dockerfile` example in `prompts/REPO_POLICIES.md` said to install lint-phase system libraries with `apk add`. The lint phase is built on the golangci-lint image, which is Debian-based and has no `apk`, and `vips-dev` is the alpine package name. The point now gives the `apt-get` command with the Debian package name (`libvips-dev`) and removes the package lists in the same `RUN`.

The other `apk` mentions are about the alpine build stage or `script/bootstrap` on the host and stay. Nothing is pinned or unpinned; that is the open owner question on #72.

Model: opus-5-5

TODO.md

@@ -21,6 +21,12 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 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`
+  image is Debian-based and has no `apk`, so the old `apk add` instruction
+  failed as written. Nothing is pinned or unpinned; that is still open on
+  issue 72.
 - 2026-10-04: Fixed the server lifecycle example in
   `prompts/GO_HTTP_SERVER_CONVENTIONS.md` (issue 86). Only fx handles SIGINT and
   SIGTERM, and `Run()` in `main` exits with the shutdown's exit code. A listen

prompts/REPO_POLICIES.md

@@ -236,8 +236,18 @@ style conventions are in separate documents:
       (e.g. a web frontend compiled in a separate stage), the lint phase must
       create placeholder files so the embed directives resolve. Example:
       `RUN mkdir -p web/dist && touch web/dist/index.html web/dist/style.css`.
-    - If the project requires CGO or system libraries for linting (e.g.
-      `vips-dev`), install them in the lint phase with `apk add`.
+    - If the project requires CGO or system libraries for linting, install them
+      in the lint phase. The `golangci/golangci-lint` image is Debian-based and
+      has no `apk`, so install with `apt-get` under the Debian package name
+      (`libvips-dev`, where alpine says `vips-dev`), and delete the package
+      lists in the same `RUN`, so the layer does not keep them:
+
+        ```dockerfile
+        RUN apt-get update \
+            && apt-get install -y --no-install-recommends libvips-dev \
+            && rm -rf /var/lib/apt/lists/*
+        ```
+
     - `.dockerignore` lets `.git` into the build context. It keeps out
       `.git/config` and each submodule's `config` under `.git/modules/` at any
       depth (`.git/modules/**/config`), which `git describe` does not need and