Add api call which uses go-container registry to create OCI images from
standard tar archives.
Consume new API when generating final images instead of docker building them
and adapts/add tests as necessary.
This change now allows to carry over xattrs to final images.
Fixes#266
The path contains an ending "/" which wouldn't match when we walk dst as
it's not there.
That had the unpleasant effect of creating empty folders in the
destinations
When we have huge file lists we can burst too much into RAM which would
cause OOMs in certain devices. Use instead a smart cache which
automatically drops to disk when necessary.
