Add an explicit log entry for when a task runner starts (#1675)

This will assist with debugging the entire logged session of a worker
with the same thread ID (as added by `_setup_logging_extra_info()`)

docker/worker/worker.py

@@ -326,6 +326,7 @@ def __init__(self, ndb_client, oss_fuzz_dir, work_dir, ssh_key_public_path,
     self._ssh_key_public_path = ssh_key_public_path
     self._ssh_key_private_path = ssh_key_private_path
     os.makedirs(self._sources_dir, exist_ok=True)
+    logging.info('Created task runner')
 
   def _git_callbacks(self, source_repo):
     """Get git auth callbacks."""