SAM run/debug: support images for 'code' debug configurations

**Desktop (please complete the following information):**

- OS: MacOS 11.6.1 (Big Sur)
- VS Code version: 1.62.1
- AWS Toolkit extension version: 1.33.0

**Question**

I'm attempting to launch a debug session for my Lambda code. I've created a Dockefile that uses the `amazon/aws-lambda-python:3.9` base image that will ultimately be used as the runtime for the Lambda function when it is uploaded to AWS.

The difficulty that I'm running into is that I don't know how to configure my `launch.json` file to use that Dockerfile when building the container that it uses to locally invoke the function, and also for VSCode to begin a debug session.

Thanks to anyone in advance for any guidance that you can provide!

Start by tracing how launch.json 'code' debug configurations select the local SAM runtime and how the referenced Dockerfile or image is handled. Done means a code debug configuration can use the Dockerfile-built image for local invocation and start a VS Code debugging session.

Written by the indexing model from the issue text.