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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


'response': response
                }
                if 'request_handler' in inspect.getargspec(self.proxy.update_listener.return_response)[0]:
                    
                    
                    kwargs['request_handler'] = self
                updated_response = self.proxy.update_listener.return_response(**kwargs)
                if isinstance(updated_response, Response):
                    response = updated_response
            
            self.send_response(response.status_code)
            content_length_sent = False
            for header_key, header_value in iteritems(response.headers):
                
                if header_key.lower() not in ('transfer-encoding', 'date', 'server'):
                    self.send_header(header_key, header_value)
                    content_length_sent = content_length_sent or header_key.lower() == 'content-length'
            if not content_length_sent:
                self.send_header('Content-Length', '%s' % len(response.content) if response.content else 0)
            
            if 'Access-Control-Allow-Origin' not in response.headers:
                self.send_header('Access-Control-Allow-Origin', '*')
            if 'Access-Control-Allow-Methods' not in response.headers:
                self.send_header('Access-Control-Allow-Methods', ','.join(CORS_ALLOWED_METHODS))
            if 'Access-Control-Allow-Headers' not in response.headers:
                self.send_header('Access-Control-Allow-Headers', ','.join(CORS_ALLOWED_HEADERS))
            if 'Access-Control-Expose-Headers' not in response.headers:
                self.send_header('Access-Control-Expose-Headers', ','.join(CORS_EXPOSE_HEADERS))
            self.end_headers()
            if response.content and len(response.content):