Stop the background thread before destroying the docker async source

Otherwise we can access a half-destroyed object from the worker thread.

Author: gnosek

userspace/libsinsp/container_engine/docker_common.cpp

@@ -43,6 +43,7 @@ docker_async_source::docker_async_source(uint64_t max_wait_ms, uint64_t ttl_ms,
 
 docker_async_source::~docker_async_source()
 {
+	this->stop();
 	g_logger.format(sinsp_logger::SEV_DEBUG,
 			"docker_async: Source destructor");