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I can get variable data with {{doc.xxxxx}}. Is "doc" the only accessible construct? Can I access "os.environ"?

I know that I can build quite complex JSON Request bodies for a WebHook by exploiting template variable expansion :

{
    "token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpZCI6NywiaWF0IjoxNTg4Nzc0Mzc2LCJleHAiOjI1MjE4OTQzNzZ9.YZLN2tXq0NRNMwQK50Wx66f4xJ2-eAT3TSj9e_FLhKA",
    "doctype": "{{doc.doctype}}",
    "data": {
        "tax_id": "{{doc.tax_id}}",
        "name": "{{doc.name}}"
    }
}

I would like to know how to pull in values from elsewhere. So, not just {{doc.xxxxx}} but from {{os.environ.PATH}}, for example.

Is there a list of accessible variable collections?

The one I most need is a value from site_config.json.

Is it possible for a WebHook to “see” the contents of site_config.json and if so, how?