integration: add OCI <-> Docker roundtrip tests
integration: add OCI <-> Docker roundtrip tests

This test is just a general smoke test to make sure there are no errors
with skopeo, but also verifying that after passing through several
translation steps an OCI image will remain in fully working order.

Signed-off-by: Aleksa Sarai <a8eb12d32d6ec07cdf737175ec7d4a0f1b4e1ee7@suse.de>

mtrmac/skopeo,jwhonce/skopeo,mtrmac/skopeo,projectatomic/skopeo,projectatomic/skopeo,jwhonce/skopeo,mtrmac/skopeo