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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


self.quiet = quiet
        if self.httpd:
            self.httpd.server_close()
            self.server_stopped = True
    @classmethod
    def create_ssl_cert(cls, random=True):
        return generate_ssl_cert(SERVER_CERT_PEM_FILE, random=random)
    @classmethod
    def get_flask_ssl_context(cls):
        if USE_SSL:
            combined_file, cert_file_name, key_file_name = cls.create_ssl_cert()
            return (cert_file_name, key_file_name)
        return None
def serve_flask_app(app, port, quiet=True, host=None, cors=True):
    if cors:
        CORS(app)
    if quiet:
        logging.getLogger('werkzeug').setLevel(logging.ERROR)
    if not host:
        host = '0.0.0.0'
    ssl_context = GenericProxy.get_flask_ssl_context()
    app.config['ENV'] = 'development'
    def noecho(*args, **kwargs):
        pass
    click.echo = noecho
    app.run(port=int(port), threaded=True, host=host, ssl_context=ssl_context)
    return app