Final cachekey for stage

[cvgw]

Fixes #682
Description

Store the last cachekey generated for each stage.
If the base image for a stage is present in the map of digest and cachekeys use the retrieved cachekey instead of the base image digest in the compositecache.

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• Includes unit tests
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Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

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- Improve caching for multistage builds where FROM points to a locally build stage