How can I access the path to the current directory in an emacs directory variable file?

According to the Emacs documentation, Directory Variables apply to all files below a directory that contains an .dir-locals.el file.

How can I, in that file, set a variable to the full path that contains the file? For example:

((nil . ((indent-tabs-mode . t)
          (my-project-path **THIS_DIRECTORY**))))