Add support for non-docker backend

[aslafy-z]

https://github.com/containers/skopeo allow to pull/push images without the need for the Docker stack. Adding compatibility with skopeo would allow running sinker where no Docker daemon are available.

EDIT: Just seen that skopeo is a CLI and should not be directly integrated as is in other softs. Maybe this is the right way: https://github.com/containers/image https://pkg.go.dev/github.com/containers/image

[acastle]

👍 agree that it would be good to support generic backends. It should be possible to handle all of sinker's functions directly through the OCI.

[jpreese]

Resolved via #63