* Extend .dockerignore integration test with copies in later stages
.dockerignore should continue to apply when copying from the build context in later stages, but it currently doesn't
* Replace excluded global with passed along FileContext struct
This new FileContext struct allows much cleaner handling of context specific file exclusions.
The global excluded file state is no longer needed.
Additionally this also fixes the issue where excluded files aren't being applied for build context copies in later build stages.
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