Differential D6972 Diff 22377 contrib/gitian-builder/doc/CACHE

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				Gitian includes two caches where build-assets may be saved for use with
				subsequent builds. These caches are only enabled if the "enable_cache" key
				is enabled in the build descriptor.

				Common cache:
				All descriptors share this cache. It can be useful for storing fetched sources,
				sharing build assets between descriptors, etc.

				Per-descriptor cache
				Files installed to this cache can only be seen by this descriptor. Use this to
				store assets created as part of the build process, to avoid having to rebuild
				them in future builds.

				If caching is enabled, GBUILD_CACHE_ENABLED will be set to 1 in the build
				script. In addition, GBUILD_PACKAGE_CACHE and GBUILD_COMMON_CACHE will be set
				to their respective paths. GBUILD_PACKAGE_CACHE contains the descriptor's
				"name" key as a means of separating caches. To add or update the cached files,
				copy them to these paths.

				Before each build, all files and folders in the cache directories will be
				transferred to the VM. After each successful build, the caches will be
				transferred back to the builder (overwriting existing files as necessary).

				It is entirely up to the user and the descriptors to verify that cached files
				are actually suitable for re-use. Overwriting existing cached files is _highly_
				discouraged, as that would likely lead to non-deterministic results. Unique
				filenames should always be used.