# This is a ...

[x] bug report
[ ] feature request

# Detailed description

When running an `serverless-express` API in a lambda pushed to localstack in Docker, OPTIONS requests from a client with the `withCredentials` flag fail because of the wildcard `'*'` in `Access-Control-Allow-Origin` header returned by calling the endpoint (e.g.: `http://localhost:4566/restapis/<id>/<stage>/_user_request_/api/`). The request doesn't reach the lambda at all.

docker-compose.yml:
```yml

services:
  localstack:
    container_name: "${LOCALSTACK_DOCKER_NAME-localstack_main}"
    image: localstack/localstack:latest
    network_mode: bridge
    ports:
      - "4566:4566"
      - "4571:4571"
    environment:
      - DEFAULT_REGION=us-east-1
      - SERVICES=iam,lambda,apigateway,s3,ses,sts,sns,cloudformation,cloudwatch
      - DEBUG=1
      - DATA_DIR=/tmp/localstack/data
      - LAMBDA_EXECUTOR=docker-reuse
      - LAMBDA_REMOTE_DOCKER=0
      - KINESIS_ERROR_PROBABILITY=${KINESIS_ERROR_PROBABILITY- }
      - DOCKER_HOST=unix:///var/run/docker.sock
      - HOST_TMP_FOLDER=${PWD}/.localstack
      - LAMBDA_DOCKER_NETWORK=bridge
    volumes:
      - "${PWD}/.localstack:/tmp/localstack"
      - "/var/run/docker.sock:/var/run/docker.sock"

```

## Expected behavior

Allow defining a single(?) allowed origin for the header so that the preflight request doesn't fail.

## Actual behavior

Browser errors out with: 

```
Access to XMLHttpRequest at '<endpoint>' from origin 'http://localhost:8080' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: The value of the 'Access-Control-Allow-Origin' header in the response must not be the wildcard '*' when the request's credentials mode is 'include'. The credentials mode of requests initiated by the XMLHttpRequest is controlled by the withCredentials attribute.
```

# Steps to reproduce

- Create a simple API based on `serverless-express` with `express-session` as session handler.
- Use `withCredentials` flag to send requests to the API via `axios` or similar

## Command used to start LocalStack

`docker-compose up`

## Client code (AWS SDK code snippet, or sequence of "awslocal" commands)


""" This interceptor method is called by the proxy when returning a response
            (*after* having forwarded the request and received a response from the backend
            service). It receives details of the incoming request as well as the response
            from the backend service, and returns either of the following results:
            * An instance of requests.models.Response to return to the client instead of the
              actual response returned from the backend service.
            * Any other value, in which case the response from the backend service is
              returned to the client.
        """
        return None
    def get_forward_url(self, method, path, data, headers):
        """ Return a custom URL to forward the given request to. If a falsy value is returned,
            then the default URL will be used.
        """
        return None
class RegionBackend(object):
    """ Base class for region-specific backends for the different APIs. """
    @classmethod
    def get(cls, region=None):