server.go: Fix rootRepo pop in ImageCopy

The previous version was invalid Go (can't index the rootRepo map with
an integer).  It's possible that there is some slicker Go syntax for
this action (Python used dict.pop()), but I don't know what it is.
image.go: Stub out ImageCopy

This currently copies the whole image, ignoring the 'resource'
argument.  I need to figure out the tar syntax to extract a sub-path,
or mimic ContainerCopy and find the mounted version of the image.