use containerWorkspaceFolder var and pass it to setup.sh (#12)

- folder name inside dev container is same as folder name on host
  so if it differs from qmk_userspace then setup.sh cannot be found
  and it sets incorrect user.overlay_dir
- docs https://containers.dev/implementors/json_reference/

.devcontainer/devcontainer.json

@@ -17,7 +17,7 @@
             ]
         }
     },
-    "postCreateCommand": "/workspaces/qmk_userspace/.devcontainer/setup.sh"
+    "postCreateCommand": "${containerWorkspaceFolder}/.devcontainer/setup.sh ${containerWorkspaceFolder}"
 
     // Features to add to the dev container. More info: https://containers.dev/features.
     // "features": {},