feat: convert container runtime from Apple Container to Docker (#323)

Swap container-runtime.ts to the Docker variant:
- CONTAINER_RUNTIME_BIN: 'container' → 'docker'
- readonlyMountArgs: --mount bind,readonly → -v host:container:ro
- ensureContainerRuntimeRunning: container system status → docker info
- cleanupOrphans: Apple Container JSON format → docker ps --filter
- build.sh default: container → docker

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

container/build.sh

@@ -8,7 +8,7 @@ cd "$SCRIPT_DIR"
 
 IMAGE_NAME="nanoclaw-agent"
 TAG="${1:-latest}"
-CONTAINER_RUNTIME="${CONTAINER_RUNTIME:-container}"
+CONTAINER_RUNTIME="${CONTAINER_RUNTIME:-docker}"
 
 echo "Building NanoClaw agent container image..."
 echo "Image: ${IMAGE_NAME}:${TAG}"