add test

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
ignore lchown error on docker cp

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #5954 from vieux/fix_cp_error

Ignore lchown error on docker cp