Cache keys don't match for builds with a cached copy command

[cvgw]

Actual behavior
When a build contains a cached layer using the COPY command all following layers in the build will produce incorrect cache keys. Specifically, the cache key used to read from the cache and the cache key used to write to the cache will be different for each layer following the copy command.

Layers affected by this will never be able to read from the cache because the key used to write the layer to the cache is different than the key used to read from the cache.

The cause of this bug is due to CachingCopyCommand and CopyCommand returning different values for FilesUsedFromContext. The result of FilesUsedFromContext is added to the cache key.

Expected behavior
All commands always produce the same cache key during read and during write given the same input.

To Reproduce
Steps to reproduce the behavior:

1. Any build with a cached copy command followed by any other caching command
2. Observe that commands following the cached copy command are never cached

Additional Information

• Dockerfile

FROM ubuntu:16.04
COPY foo.txt bar.txt
RUN apt-get update -y

• Build Context
  echo 'meow' > foo.txt

Triage Notes for the Maintainers

Description	Yes/No
Please check if this a new feature you are proposing
Please check if the build works in docker but not in kaniko
Please check if this error is seen when you use --cache flag
Please check if your dockerfile is a multistage dockerfile