Reset SystemContext.DockerAuthConfig when using mirrors at different registries

[mtrmac]

This was brought up sometime during the reviews, but was apparently lost.

Consider registries.conf:

[[registry]]
location = "example.com"
mirror = [
	{ location = "docker.io" },
	{ location = "registry.access.redhat.com" },
	{ location = "registry.redhat.io" },
	{ location = "registry.fedoraproject.org" },
]

Then

skopeo --registries-conf registries.conf copy --src-creds foo:bar docker://example.com/foo/bar dir:t

currently sends the foo:bar to every mirror that requires authentication, not just to example.com; that seems clearly undesirable.

So, clear SystemContext.DockerAuthConfig when using a mirror at a different registry than the one specified by the user; if the mirrors require credentials, they have to be set up in the auth file, e.g. by podman login.

@vrothberg @saschagrunert PTAL.

(#588 may be relevant for more discussion about DockerAuthConfig)

[saschagrunert]

LGTM, but the CI seems to need a rebump. Maybe a test case in skopeo or a unit test here would be good to add.

[vrothberg]

LGTM. I opened #627 to make CI pass again. ubuntu:artful (i.e., 17:10) hit EOL.