Better guidance for using `amazon/aws-codebuild-local`

I've been trying to use `amazon/aws-codebuild-local` to debug some build failures, and it's not clear at all how the environment variables are used. Some guidance and examples would be really welcome. In particular:

* `IMAGE` - Took a long time for me to realize I had to build my own local version of the codebuild images in order to specify a value here. I thought I could use the codebuild images (`aws/codebuild/standard:2.0`) but obviously doesn't work.
* `SOURCE` - Does codebuild expect to download my source from GitHub or similar, or can I just mount it into the container?
* aws credentials - Does the local container need my credentials for anything? How do I pass them along?

Finally, I'm on Windows and the fact that `codebuild_build.sh` is a bash script is painful. I've had to reverse engineer it to figure out how it is calling the `amazon/aws-codebuild-local` image.

Start with the referenced codebuild_build.sh script and the amazon/aws-codebuild-local image invocation. Document how IMAGE, SOURCE, and AWS credentials are used, including mounting local source, and explain a practical Windows alternative to the Bash script; done means the guidance includes examples for each case.

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