This will return a string representaiton of the current filesystem to be
used with caching.
Whenever a file is explictly added (via ADD or COPY), it will be stored
in "added" in the LayeredMap. The file will map to a hash created by
CacheHasher (which doesn't take into account mtime, since that will be
different with every build, making the cache useless)
Key() will returns a sha of the added files which will be used in
determining the overall cache key for a command.
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