* Rework cache key generation a bit.
Cache keys are now based on the previous commands, rather than the previous state
of the filesystem.
* Refactor command interface a bit, only cache the context for commands that use it.
CacheCommand returns true if the command should be cached. Currently,
it's only true for RUN but can be added to ADD/COPY later on (these are
different since the contents of files for ADD/COPY need to be included
in the cache key generation).
I also changed CreatedBy to String so that we can log each command
before cache extraction or regular execution takes place.
* 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.
Don McCasland
dlorenc
Priya Wadhwa
Nick Kubala
Jon Johnson
Matt Rickard