I found and upvoted this solution on ServerFault.

TLDR:

  1. Use OS X Disk Utility to Create a New Sparse, Case Sensitive Disk
  2. Configure VirtualBox to use this new disk

Some screenshots that may be helpful:

From Disk Utility -> File -> New Image:Screen Shot 2017-09-06 at 13.37.52

From VirtualBox -> Preferences: Screen Shot 2017-09-06 at 13.37.14