test(dind): assert public-mode passthrough actually copies a public host image (issue #96)

The host-image passthrough test only asserted the negative path for the
default public mode: a locally-built fixture (no RepoDigest) is skipped.
The positive path the feature exists for - a genuinely public image
(carrying a RepoDigest from an allowlisted registry) IS copied into the
inner daemon - was structurally untested. The throwaway host daemon was
seeded with only the offline fixture, so public mode had no eligible
image at all and a 'public copies nothing' regression would ship green.

Seed the throwaway host daemon with a real pulled alpine:3.20 (pulling is
what records a RepoDigest; docker save/load does not preserve one) and
assert in the public-mode block that the image lands in the inner daemon
and is logged as loaded. Closes the false-positive gap downstream
(link-assistant/hive-mind#1879) relies on.

Also drop the stray root .gitkeep PR-creation artifact.

Fixes #96

Author: konard

.changeset/issue-96-public-passthrough-positive-assertion.md

@@ -0,0 +1,5 @@
+---
+bump: patch
+---
+
+dind-box: close a false-positive coverage gap in the host-image passthrough test (issue #96). `tests/dind/example-preload-images.sh` previously only asserted that `public` mode skips a locally-built fixture (no RepoDigest); it never asserted the positive path — that a genuinely public image (carrying a RepoDigest from an allowlisted registry) IS copied into the inner daemon. The throwaway host daemon is now also seeded with a real pulled `alpine:3.20`, and the `public`-mode block asserts that image lands in the nested daemon and is logged as loaded. A "public copies nothing" regression — the exact symptom downstream (`link-assistant/hive-mind#1879`) relies on not happening — now fails CI instead of shipping green.

.gitkeep

@@ -145,6 +145,17 @@ log "throwaway host daemon is ready"
 docker exec -i "$host_daemon_container" \
   docker -H unix:///sockets/docker.sock load < "$tarball_dir/image.tar"
 
+# Also seed it with a genuinely public image. Pulling it from a public registry
+# is what records a RepoDigest (docker save/load does NOT preserve one), so this
+# is the "freely re-pullable" case the default public mode MUST pass through.
+# This is the positive counterpart to the fixture: without it, public mode has no
+# eligible image and a "public copies nothing" regression would ship green.
+public_image="alpine:3.20"
+log "pulling a real public image (${public_image}) into the throwaway host daemon"
+if ! $host_docker pull "$public_image" >/dev/null; then
+  fail "could not pull ${public_image} into the throwaway host daemon (network required)"
+fi
+
 # all mode: every tagged host image is copied, including this local fixture.
 log "starting consumer with DIND_HOST_PASSTHROUGH=all"
 run_dind_container "$all_container" \
@@ -174,6 +185,19 @@ if ! docker logs "$public_container" 2>&1 | grep -q "host-image passthrough (mod
   docker logs "$public_container" >&2 || true
   fail "expected the consumer to run host-image passthrough in public mode"
 fi
-log "public-mode passthrough correctly skipped the local fixture (security filter held)"
+# Positive assertion: a host image carrying a RepoDigest from an allowlisted
+# public registry MUST land in the inner daemon. This is the behavior downstream
+# relies on (link-assistant/hive-mind#1879) and the path the suite previously
+# left structurally untested, so a "public copies nothing" regression now fails.
+if ! docker exec "$public_container" docker image inspect "$public_image" >/dev/null 2>&1; then
+  docker logs "$public_container" >&2 || true
+  docker exec "$public_container" docker images >&2 || true
+  fail "public mode must pass through a host image that has a public RepoDigest (${public_image})"
+fi
+if ! docker logs "$public_container" 2>&1 | grep -q "passthrough loading host image: ${public_image}"; then
+  docker logs "$public_container" >&2 || true
+  fail "expected public mode to log loading the public host image (${public_image})"
+fi
+log "public-mode passthrough copied the public image and skipped the local fixture (security filter held)"
 
 log "preload example passed"