test-build: drop the superseded image after --pull

A --pull that moves the tag leaves the previous image untagged, ~23G
for current. Remove that specific image right after a successful pull
instead of relying on a host-wide prune: other dangling images on the
host are left alone, and rmi refuses if a concurrent run still uses it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 13 insertions, 1 deletions

diff --git a/test-build b/test-build
index 56cc291..f719a73 100755
--- a/test-build
+++ b/test-build
@@ -861,8 +861,20 @@ print_summary() {
 # so an up-to-date image costs one manifest request.
 require_image() {
   if [[ $PULL_IMAGE -eq 1 ]]; then
+    local old_id new_id
+    old_id="$(docker image inspect --format '{{.Id}}' "$ACTIVE_IMAGE" 2>/dev/null || true)"
     echo "Pulling: $ACTIVE_IMAGE" >&2
-    docker pull "$ACTIVE_IMAGE" >&2 && return 0
+    if docker pull "$ACTIVE_IMAGE" >&2; then
+      # A pull that moved the tag leaves the old image untagged (~23G for
+      # current). Drop that one image, not every dangling one on the host;
+      # rmi refuses if a container (e.g. a concurrent run) still uses it.
+      new_id="$(docker image inspect --format '{{.Id}}' "$ACTIVE_IMAGE")"
+      if [[ -n $old_id && $old_id != "$new_id" ]]; then
+        docker rmi "$old_id" >&2 \
+          || echo "WARN: superseded image $old_id still in use; left in place." >&2
+      fi
+      return 0
+    fi
     if docker image inspect "$ACTIVE_IMAGE" >/dev/null 2>&1; then
       echo "WARN: pull failed; using the local copy, which may be stale." >&2
       return 0