Debug error at start session: "Variable ${workspaceFolder} can not be resolved in a multi folder workspace. Scope this variable using ':' and a workspace folder name"

Workspace file contains:

{
  "folders": [
    { "path": "." },
    { "path": "../some-folder" }
  ],
  "extensions": {
    "recommendations": [
      "ms-dotnettools.csdevkit",
      "csharpier.csharpier-vscode"
    ]
  },
  "settings": { ... },
  "launch": {
    "version": "0.2.0",
    "configurations": [
      {
        "name": "SomeService (docker attach)",
        "type": "docker",
        "request": "attach",
        "platform": "netCore",
        "containerName": "some-service",
        "processName": "Some.Service.Host",
        "sourceFileMap": {
          "/": "~",
        }
      },
      ...
    ]
  }
}

Expected the debug session to start without errors.

Source of issue:
https://github.com/microsoft/vscode-docker/blob/v1.29.2/src/debugging/netcore/NetCoreDebugHelper.ts#L175

Start at src/debugging/netcore/NetCoreDebugHelper.ts around line 175 and inspect how ${workspaceFolder} is resolved for the reported multi-folder workspace. Reproduce the Docker attach configuration and verify that the debug session starts without the variable-resolution error.