Fix the bug that when step failed, the EndStep passed in a nil error(result of docker inspect command), it should pass in a non-nil error instead, because if EndStep pass in a non-nil error, the step status will be Error, which is critical for the following analysis.
Fix the bug that when step failed, the EndStep passed in a nil error(result of docker inspect command), it should pass in a non-nil error instead, because if
EndSteppass in a non-nil error, the step status will beError, which is critical for the following analysis.