During docker build, host files can be passed to the docker build through
docker context files. But there is no straightforward way to transfer
the files from docker build to host.
This feature provides a tricky way to pass the cache contents from docker
build to host. It tar's the cached content and encodes them as base64 format
and passes it through a log file with a special tag as 'VCSTART and VCENT'.
Slave.mk in the host, it extracts the cache contents from the log and stores them
in the cache folder. Cache contents are encoded as base64 format for
easy passing.
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#### Why I did it
#### How I did it
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Why I did it
Unify the Debian mirror sources
Make easy to upgrade to the next Debian release, not source url code change required.
Support to customize the Debian mirror sources during the build
Relative issue: #12523
