It is my understanding that setting an environment varialbe `TEST_AWS_ACCOUNT_ID` for localstack sets that as the AWS account for the user in localstack.

However, setting it does not actually change any ARNs. For example:

1. Start localstack using `TEST_AWS_ACCOUNT_ID="111111111111" DEBUG=1 localstack start`
2. Deploy anything with an ARN (in this example I deployed a lambda)
3. Check the ARN. Example output for lambda:
```json
$ awslocal lambda list-functions
{
    "Functions": [
        {
            "FunctionName": "localtest-local-hello",
            "FunctionArn": "arn:aws:lambda:us-east-1:000000000000:function:localtest-local-hello",
            "Runtime": "nodejs10.x",
            "Role": "test_role",
            "Handler": "handler.hello",
            "CodeSize": 3077,
            "Description": "",
            "Timeout": 6,
            "MemorySize": 1024,
            "LastModified": "2019-11-06T06:43:53.451+0000",
            "CodeSha256": "V77gR13o90ET7+XquXBBc3Rxr1qDu1bE7S+A0Wbx5ys=",
            "Version": "$LATEST",
            "TracingConfig": {
                "Mode": "PassThrough"
            },
            "RevisionId": "3d4ed679-c1a9-4b7e-ad9e-f27e8543c5e9"
        }
    ]
}
```

ARN is still 000000000000
func_arn = uri.split(':lambda:path')[1].split('functions/')[1].split('/invocations')[0]
            data_str = json.dumps(data) if isinstance(data, (dict, list)) else data
            account_id = uri.split(':lambda:path')[1].split(':function:')[0].split(':')[-1]
            source_ip = headers['X-Forwarded-For'].split(',')[-2]
            
            
            request_context = {
                'path': relative_path,
                'accountId': account_id,
                'resourceId': resource.get('id'),
                'stage': stage,
                'identity': {
                    'accountId': account_id,
                    'sourceIp': source_ip,
                    'userAgent': headers['User-Agent'],
                }
            }
            try:
                path_params = extract_path_params(path=relative_path, extracted_path=extracted_path)
            except Exception:
                path_params = {}
            result = lambda_api.process_apigateway_invocation(func_arn, relative_path, data_str,
                headers, path_params=path_params, query_string_params=query_string_params,
                method=method, resource_path=path, request_context=request_context)
            if isinstance(result, FlaskResponse):
                return flask_to_requests_response(result)
            if isinstance(result, Response):
                return result
            response = Response()
            parsed_result = result if isinstance(result, dict) else json.loads(result)