Manual:$wgFileCacheDirectory

Details
If you switch on server-side file caching , then you can use this variable to specify where the cached pages will be saved on your server filesystem.

It's an absolute path, so you could specify somewhere under for example:

...but you can specify this absolute path in relation to other directory path variables, e.g.:

This specifies a "cache" directory under the image uploads directory (normally called "images"). This is the default setting in fact.