This question is a bit old now but I found it first on google searching for the same thing.

The setting that's worked for me as of Nov 2023 is:
devcontainer.json

{
    "name": "python dev",
    "image": "python:3.10",
    "customizations":{
        "vscode": {
            "extensions":[
                "ms-python.python",
                "ms-python.vscode-pylance"
            ],
            "settings": {
                "python.defaultInterpreterPath": "/usr/local/bin/python"
            }
        }
    }
}

In case that's not enough, I also have "python.defaultInterpreterPath": "/usr/local/bin/python", copied into .vscode/settings.json just to be certain.

I often have this same issue with Python+Devcontainer. What works for me is:

1. setting this env variable: export PYTHONPATH=/workspace, you can also try with /workspaces or /yourProjectName depending on your setup.

2. restart the Python Language Server with Ctrl+Shift+P > Python: Restart Language Server.

Note that you do not need to rebuild the container. When you find the correct value for PYTHONPATH, you can add it to your Devcontainer configuration.