This has a few benefits:
* The shellcheck SC2086 warning is disabled for the whole command, but
is only needed for the DOCKER_BUILD_CACHE_ARG env var. So in Python,
only pass this one env var to shlex.split() for proper word splitting.
* Future logic improvements can be implemented in Python.
The comments are moved, which can be checked via the git options:
--color-moved=dimmed-zebra --color-moved-ws=ignore-all-space
fab64a5d6f