Currently the default WorkingDir in test is "/", while in reallife it's
empty.
This change the tests to reflect reel life and fix the case where
First WorkingDir is relative.
* Vendor changes for go-containerregistry switch.
* Manual changes for go-containerregistry switch.
The biggest change is refactoring the tarball unpacking.
* Pull more of container-diff out.
* More vendor removals.
* More unit tests.