This patch adds the option to use mirrors for the docker registries in trivy.
This way it is possible to download the images from self-hosted mirrors instead
of e.g. index.docker.io. This has some benefits, like not being hit by download
restrictions.

* Use a single configuration key to describe the registry as well as the
  mirror.
* More meaningful error inspection in tests.
* Use github.com/google/go-containerregistry to parse image reference.