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We have several Kinesis streams set up with event notifications that trigger lambdas.  When the LAMBDA_EXECUTOR is set to 'docker' and we send a large number or records to Kinesis, multiple Lambdas will execute, but currently they are failing to create with error:

```
Exception: Lambda process returned error status code: 1. Output:
time="2019-09-18T17:28:11Z" level=error msg="error getting events from daemon: net/http: request canceled" 
Error response from daemon: driver failed programming external connectivity on endpoint busy_sanderson (a78e45a41d0ce5b0d8bab6ff54344fc34fe2b4969f55bf568e753121abc8759e): Bind for 0.0.0.0:38763 failed: port is already allocated
```

I believe this is due to the new debug_java_port introduced with this PR https://github.com/localstack/localstack/pull/1523

With this change, you are setting the port in the constructor by using a method known to have a race condition (https://github.com/localstack/localstack/blob/master/localstack/utils/common.py#L354)
(https://gist.github.com/gabrielfalcao/20e567e188f588b65ba2)

This should be changed to ensure unique ports are being issued across all lambdas


proxy_url = '%s%s' % (forward_url, path)
        target_url = self.path
        if '://' not in target_url:
            target_url = '%s%s' % (forward_url, target_url)
        
        if not forward_headers.get('Host'):
            forward_headers['host'] = urlparse(target_url).netloc
        if 'localhost.atlassian.io' in forward_headers.get('Host'):
            forward_headers['host'] = 'localhost'
        forward_headers['X-Forwarded-For'] = self.build_x_forwarded_for(forward_headers)
        try:
            response = None
            modified_request = None
            
            for listener in self._listeners():
                if not listener:
                    continue
                listener_result = listener.forward_request(method=method,
                    path=path, data=data, headers=forward_headers)
                if isinstance(listener_result, Response):
                    response = listener_result
                    break
                if isinstance(listener_result, dict):
                    response = Response()
                    response._content = json.dumps(listener_result)
                    response.status_code = 200
                    break
                elif isinstance(listener_result, Request):
                    modified_request = listener_result
                    data = modified_request.data