Fix git ownership and -race in the canonical Go Dockerfile (closes #73)

The test phase ran go test -race on the alpine Go image, which has no C
compiler, so cgo was off and the phase failed with "-race requires cgo"
before running a test. It now uses the Debian Go image.

A build context sent as a tar stream keeps the sender's file owners, so
git in the stage that compiles refused the checkout and the version came
out empty. That stage now runs
git config --system --add safe.directory /src, and the policy text and
both checklists say why. The apk add --no-cache git line is unchanged:
its pinning waits on #72.

Model: opus-5-5

TODO.md

@@ -21,6 +21,13 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-03: Fixed two defects in the canonical Go `Dockerfile` example (issue
+  73). The test phase now uses the Debian Go image, since `-race` needs cgo and
+  the alpine image has no C compiler, so the phase failed before running a test.
+  The stage that compiles runs `git config --system --add safe.directory /src`,
+  because a context sent as a tar stream keeps the sender's file owners and git
+  refuses that checkout, leaving the version empty. Both checklists state that
+  step in the same words.
 - 2026-10-03: Brought the canonical `.gitignore` level with `.dockerignore` on
   secrets (issue 38): it now also ignores `prod.env`-style `*.env` files,
   `.envrc`, `*.p12`, `*.pfx` and the extensionless SSH private keys, written to

prompts/EXISTING_REPO_CHECKLIST.md

@@ -1,6 +1,6 @@
 ---
 title: Existing Repo Checklist
-last_modified: 2026-10-02
+last_modified: 2026-10-03
 ---
 
 Use this checklist when beginning work in a repo that may not yet conform to our
@@ -67,10 +67,14 @@ with your task.
       argument when one is given, otherwise from `git describe --tags --always`.
       That gives the tag on a tagged commit; on a later commit, the tag, the
       number of commits since it and the short commit (`v1.2.3-4-gabc1234`); and
-      the short commit when no tag is reachable. `ARG VERSION` has no default,
-      and the build fails if the context carries `.git` and the version still
-      comes out empty, `dev` or `unknown`. A plain `docker build .` with no
-      build arguments must succeed; a Dockerfile that refuses an empty build
+      the short commit when no tag is reachable. The stage that compiles also
+      marks its working directory safe for git
+      (`git config --system --add safe.directory /src`): a context sent as a tar
+      stream keeps the sender's file owners, and git refuses a checkout owned by
+      another user, so the version would come out empty. `ARG VERSION` has no
+      default, and the build fails if the context carries `.git` and the version
+      still comes out empty, `dev` or `unknown`. A plain `docker build .` with
+      no build arguments must succeed; a Dockerfile that refuses an empty build
       argument drops that refusal and keeps the argument. `script/docker` and
       `script/cibuild` pass the version they compute on the host; it takes
       precedence. A tag-derived version additionally needs `fetch-depth: 0` on

prompts/NEW_REPO_CHECKLIST.md

@@ -1,6 +1,6 @@
 ---
 title: New Repo Checklist
-last_modified: 2026-10-02
+last_modified: 2026-10-03
 ---
 
 Use this checklist when creating a new repository from scratch. Follow the steps
@@ -76,10 +76,14 @@ Template files can be fetched from:
       argument when one is given, otherwise from `git describe --tags --always`.
       That gives the tag on a tagged commit; on a later commit, the tag, the
       number of commits since it and the short commit (`v1.2.3-4-gabc1234`); and
-      the short commit when no tag is reachable. `ARG VERSION` has no default,
-      and the build fails if the context carries `.git` and the version still
-      comes out empty, `dev` or `unknown`. A plain `docker build .` with no
-      build arguments must succeed; a Dockerfile that refuses an empty build
+      the short commit when no tag is reachable. The stage that compiles also
+      marks its working directory safe for git
+      (`git config --system --add safe.directory /src`): a context sent as a tar
+      stream keeps the sender's file owners, and git refuses a checkout owned by
+      another user, so the version would come out empty. `ARG VERSION` has no
+      default, and the build fails if the context carries `.git` and the version
+      still comes out empty, `dev` or `unknown`. A plain `docker build .` with
+      no build arguments must succeed; a Dockerfile that refuses an empty build
       argument drops that refusal and keeps the argument.
     - The Dockerfile carries a `lint` phase and a `test` phase, each invoking
       its tool directly rather than through `make` or `script/`, and the final

prompts/REPO_POLICIES.md

@@ -1,6 +1,6 @@
 ---
 title: Repository Policies
-last_modified: 2026-10-02
+last_modified: 2026-10-03
 ---
 
 This document covers repository structure, tooling, and workflow standards. Code
@@ -160,7 +160,7 @@ style conventions are in separate documents:
 - **The gate phases are separate stages, and the build stage depends on both.**
   The lint phase is based on the `golangci/golangci-lint` image (pinned by
   hash), so lint failures surface in seconds rather than after a full compile,
-  and the test phase is based on the Go image. The canonical Go repo
+  and the test phase is based on the Debian Go image. The canonical Go repo
   `Dockerfile`:
 
     ```dockerfile
@@ -173,8 +173,9 @@ style conventions are in separate documents:
     COPY . .
     RUN golangci-lint run --config .golangci.yml ./...
 
-    # Test phase
-    # golang:1.x-alpine, YYYY-MM-DD
+    # Test phase. -race needs cgo and so a C compiler, which the Debian Go
+    # image ships and the alpine one does not.
+    # golang:1.x, YYYY-MM-DD
     FROM golang@sha256:... AS test
     WORKDIR /src
     COPY go.mod go.sum ./
@@ -192,6 +193,8 @@ style conventions are in separate documents:
     COPY --from=lint /src/go.sum /dev/null
     COPY --from=test /src/go.sum /dev/null
     RUN apk add --no-cache git
+    # A tar-stream context keeps the sender's file owners, which git refuses.
+    RUN git config --system --add safe.directory /src
     WORKDIR /src
     COPY go.mod go.sum ./
     RUN go mod download
@@ -244,11 +247,14 @@ style conventions are in separate documents:
       `git describe --tags --always`. That gives the tag on a tagged commit; on
       a later commit, the tag, the number of commits since it and the short
       commit (`v1.2.3-4-gabc1234`); and the short commit when no tag is
-      reachable. `ARG VERSION` has no default, and the build fails if the
-      context carries `.git` and the version still comes out empty, `dev` or
-      `unknown`. A plain `docker build .` with no build arguments must succeed;
-      a Dockerfile that refuses an empty build argument drops that refusal and
-      keeps the argument.
+      reachable. The stage that compiles also marks its working directory safe
+      for git (`git config --system --add safe.directory /src`): a context sent
+      as a tar stream keeps the sender's file owners, and git refuses a checkout
+      owned by another user, so the version would come out empty. `ARG VERSION`
+      has no default, and the build fails if the context carries `.git` and the
+      version still comes out empty, `dev` or `unknown`. A plain
+      `docker build .` with no build arguments must succeed; a Dockerfile that
+      refuses an empty build argument drops that refusal and keeps the argument.
 
 - Every repo should have a Gitea Actions workflow (`.gitea/workflows/`) that
   runs `script/cibuild` on push, and checks out the repo as its only other step.