Summary:
- Fix a few shellcheck warnings on `docker_build_with_ccache.sh`.
- This will help when new changes are made to this script as
  shellcheck is run internally.
Pull Request resolved: https://github.com/facebook/folly/pull/1054

Reviewed By: yns88

Differential Revision: D14454554

Pulled By: yfeldblum

fbshipit-source-id: f4b692f21d2478f9db06b478ce56c1ba57fa59e0