Added a --ignore flag to ignore packages and files in the build context.
This should mimic the .dockerignore file. Before starting the build, we
go through and delete ignored files from the build context.

It seems like .dockerignore deletes files containeed within the file locally upon
docker build, so I created a temporary test dir to make sure the
--ignore flag works. We make sure it exists before building docker and
kaniko for Dockerfile_test_ignore, and then delete it after the builds
have completed.