[18.09 backport] Fix docker cp when container source path is / Upstream-commit: c513a4c6c2989445efb75f84b5f62440d1a7bf0c Component: engine
[18.09 backport] Fix docker cp when container source path is / Upstream-commit: c513a4c6c2989445efb75f84b5f62440d1a7bf0c Component: engine