I have a base task for each pipeline step. When I initialize a pipeline, for each step I clone the corresponding task, modify it and add it as a step. Tasks are launched from a pipeline, not cli. I'm absolutely sure docker argument is not empty (I specify it with export_data['container']['image'] = ' http://registry.gitlab.com/cherrylabs/ml/clearml-demo:clearml ' , and it shows on Web UI)