Is it possible to reuse `tern.utils.rootfs.get_working_directory()` for cache dir?

[kairoaraujo]

The cache directory can be retrieved from tern using tern.utils.rootfs.get_working_directory(). You may also want to add a note in the README about setting the TERN_DEFAULT_REGISTRY value when not working with the docker container.

Originally posted by @nishakm in #3 (comment)