Add Docker-backed integration test rig

scripts/test-rig/ensure-image.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+IMAGE_TAG="${SANDBOX_AGENT_TEST_IMAGE:-sandbox-agent-test:dev}"
+LOCK_DIR="$ROOT_DIR/.context/docker-test-image.lock"
+
+release_lock() {
+  if [[ -d "$LOCK_DIR" ]]; then
+    rm -rf "$LOCK_DIR"
+  fi
+}
+
+while ! mkdir "$LOCK_DIR" 2>/dev/null; do
+  sleep 1
+done
+
+trap release_lock EXIT
+
+docker build \
+  --tag "$IMAGE_TAG" \
+  --file "$ROOT_DIR/docker/test-agent/Dockerfile" \
+  "$ROOT_DIR" \
+  >/dev/null
+
+printf '%s\n' "$IMAGE_TAG"