Use dirImageDestination for writing to local files in docker.go
Use dirImageDestination for writing to local files in docker.go

This will hopefully allow better reuse of the "copy images" code from
docker.go in the future.

No behavior change, the dirImageDestination code was based on the code
this commit is replacing.

containers/image,masters-of-cats/image,mikebrow/skopeo,projectatomic/skopeo,jwhonce/skopeo,projectatomic/skopeo,mtrmac/skopeo,containers/image,mikebrow/skopeo,mtrmac/skopeo,jwhonce/skopeo,mtrmac/skopeo