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Having upgraded to the latest LocalStack version, I've noticed that Docker containers from Lambda executions are immediately removed when they terminate.

This was added in https://github.com/localstack/localstack/pull/1584

I can appreciate the need to clean up containers, however this feature of having them stay up is actually very useful for debugging things when they go wrong!

Would it be possible to have a flag so that when running LocalStack we can specify whether to remove containers on completion? Something like:

```
services:
  localstack:
    image: localstack/localstack:latest
    ports:
      - "4567-4593:4567-4593"
    environment:
      - SERVICES=serverless,ssm,stepfunctions
      - DOCKER_HOST=unix:///var/run/docker.sock
      - LAMBDA_EXECUTOR=docker
      - REMOVE_CONTAINERS_ON_LAMBDA_TERMINATE=true #(or false)
    volumes:
      - "${TMPDIR:-/tmp/localstack}:/tmp/localstack"
      - "/var/run/docker.sock:/var/run/docker.sock"
```

Thanks for the great work folks!
def do_execute(*args):
            
            invocation_time = int(time.time() * 1000)
            
            try:
                result, log_output = self._execute(func_arn, func_details, event, context, version)
            finally:
                self.function_invoke_times[func_arn] = invocation_time
            
            self._store_logs(func_details, log_output, invocation_time)
            
            return result, log_output
        
        if asynchronous:
            LOG.debug('Lambda executed in Event (asynchronous) mode, no response from this '
                      'function will be returned to caller')
            FuncThread(do_execute).start()
            return None, 'Lambda executed asynchronously.'
        return do_execute()
    def _execute(self, func_arn, func_details, event, context=None, version=None):
        """ This method must be overwritten by subclasses. """
        raise Exception('Not implemented.')
    def startup(self):
        pass
    def cleanup(self, arn=None):
        pass
    def _store_logs(self, func_details, log_output, invocation_time):
        if not aws_stack.is_service_enabled('logs'):