I'm trying to use the vscode debugger attach to a process inside a docker container

I'm trying to follow these instructions: https://code.visualstudio.com/docs/containers/debug-python

I am inside the container in vscode. Under "Additional configurations" I created this config in launch.json:

"configurations": [
        {
            "name": "Python: Attach using Process Id",
            "type": "python",
            "request": "attach",
            "processId": "${command:pickProcess}",
            "justMyCode": true
        },

I go into Run and Debug, select "Python: Attach using Process Id" and run it. I then select this process: root 11 1 7 23:22 ? 00:00:14 /usr/bin/python3 /usr/local/bin/celery -A myprogram beat But this fails with: "Timed out waiting for debug server to connect"

What am I doing wrong?