Previously kaniko would compute the cache key for any copy command by computing
the combined hash of all files in a directory, even if they were listed
as ignored.
With this change, the cache key creation was updated to be aware of ignored
files.
Related issues:
* https://github.com/GoogleContainerTools/kaniko/issues/594
* use the cachekey of the src stage rather than the digest
for COPY --from commands as they are reproducible unlike digests
* track digest to cache keys and stage indexes to digest
* add extra debug logging for troubleshooting cachekey building issues
* convert Sha256 hashes to hex encoded strings rather than plain strings
for easier human reading
* 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.
Moritz Wanzenböck
Cole Wippern
dlorenc