I run task that shows ${localWorkspaceFolder} variable in Visual Studio Code. Task result shows that ${localWorkspaceFolder} is just empty. Why?

Task output:

Executing task in folder esp32_7: echo Workspace folder: ${localWorkspaceFolder} 

Workspace
folder:
 *  Terminal will be reused by tasks, press any key to close it. 

Task code:

{
          "label": "Echo Workspace Folder",
          "type": "shell",
          "command": "echo Workspace folder: ${localWorkspaceFolder}"
}

This is problem for me, because this variable is used in my ESP32 development docker container. And I’m getting error while reopening in container:

Workspace folder in devcontainer.json must be an absolute path.

Is it possible to set this variable to particular value manually?
Who is responsible on setting this variable in project?