docker save: fix filemode permissions

currently the files created are not readable. This makes the files and
directories permissions more sane.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Merge pull request #5087 from vbatts/vbatts-save_permissions

docker save: fix filemode permissions