Update macOS CI install README

ci-install/macos/README.rst

@@ -86,7 +86,9 @@ Building the reference setup
         cache_dir = "/Users/gitlab/caches"  # this is optional, but desirable
         shell = "bash"
 7. While at the gitlab user, install `Docker for Mac`_.  Ensure to set it up to
-   start at login.
+   start at login.  In "Preferences > Filesystem Sharing", ensure that
+   `/var/folders` is included in the list (that is the default location for
+   temporary files in macOS).
 8. Reboot the machine. At this point, the gitlab user should be auto-logged and
    the runner process should be executing.  Congratulations, you're done!