Avoid redundant Docker test image rebuilds

scripts/test-rig/ensure-image.sh

@@ -4,6 +4,17 @@ set -euo pipefail
 ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
 IMAGE_TAG="${SANDBOX_AGENT_TEST_IMAGE:-sandbox-agent-test:dev}"
 LOCK_DIR="$ROOT_DIR/.context/docker-test-image.lock"
+STAMP_FILE="$ROOT_DIR/.context/docker-test-image.stamp"
+
+INPUTS=(
+  "$ROOT_DIR/Cargo.toml"
+  "$ROOT_DIR/Cargo.lock"
+  "$ROOT_DIR/server"
+  "$ROOT_DIR/gigacode"
+  "$ROOT_DIR/resources/agent-schemas/artifacts"
+  "$ROOT_DIR/scripts/agent-configs"
+  "$ROOT_DIR/docker/test-agent/Dockerfile"
+)
 
 release_lock() {
   if [[ -d "$LOCK_DIR" ]]; then
@@ -11,16 +22,51 @@ release_lock() {
   fi
 }
 
+latest_input_mtime() {
+  find "${INPUTS[@]}" -type f -exec stat -f '%m' {} + | sort -nr | head -n1
+}
+
+image_is_ready() {
+  if ! docker image inspect "$IMAGE_TAG" >/dev/null 2>&1; then
+    return 1
+  fi
+
+  if [[ ! -f "$STAMP_FILE" ]]; then
+    return 1
+  fi
+
+  local stamp_mtime
+  stamp_mtime="$(stat -f '%m' "$STAMP_FILE")"
+  local latest_mtime
+  latest_mtime="$(latest_input_mtime)"
+
+  [[ -n "$latest_mtime" && "$stamp_mtime" -ge "$latest_mtime" ]]
+}
+
+mkdir -p "$ROOT_DIR/.context"
+
+if image_is_ready; then
+  printf '%s\n' "$IMAGE_TAG"
+  exit 0
+fi
+
 while ! mkdir "$LOCK_DIR" 2>/dev/null; do
   sleep 1
 done
 
 trap release_lock EXIT
 
+if image_is_ready; then
+  printf '%s\n' "$IMAGE_TAG"
+  exit 0
+fi
+
 docker build \
   --tag "$IMAGE_TAG" \
   --file "$ROOT_DIR/docker/test-agent/Dockerfile" \
   "$ROOT_DIR" \
   >/dev/null
 
+touch "$STAMP_FILE"
+
 printf '%s\n' "$IMAGE_TAG"