use containerWorkspaceFolder var and pass it to setup.sh (qmk#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": {},

.devcontainer/setup.sh

@@ -9,13 +9,15 @@ rm get-pip.py
 
 python3 -m pip install --upgrade milc
 
-git config --global --add safe.directory /workspaces/qmk_userspace
+userspacePath="$1"
+
+git config --global --add safe.directory "$userspacePath"
 git submodule update --init --recursive
 
 [ -d /workspaces/qmk_firmware ] || git clone https://github.com/qmk/qmk_firmware.git /workspaces/qmk_firmware
 git config --global --add safe.directory /workspaces/qmk_firmware
 
 qmk config user.qmk_home=/workspaces/qmk_firmware
-qmk config user.overlay_dir=/workspaces/qmk_userspace
+qmk config user.overlay_dir="$userspacePath"
 
 qmk git-submodule