Fix dockerClient use with nil SystemContext
Fix dockerClient use with nil SystemContext

This does not manifest with skopeo because skopeo usually uses
contextFromGlobalOptions() which is never nil, but it still is a
violation of the documented API (and breaks the copy-lookalike I use for
quick debugging).

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,masters-of-cats/image,containers/image