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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!
""" Return the string to be used for running Docker commands. """
        return config.DOCKER_CMD
    def prepare_event(self, environment, event_body):
        """ Return the event as a stdin string. """
        
        environment['AWS_LAMBDA_EVENT_BODY'] = event_body
        return None
    def _execute(self, func_arn, func_details, event, context=None, version=None):
        lambda_cwd = func_details.cwd
        runtime = func_details.runtime
        handler = func_details.handler
        environment = func_details.envvars.copy()
        
        if config.USE_SSL:
            environment['USE_SSL'] = '1'
        
        if not event:
            LOG.warning('Empty event body specified for invocation of Lambda "%s"' % func_arn)
            event = {}
        event_body = json.dumps(json_safe(event))
        stdin = self.prepare_event(environment, event_body)
        docker_host = config.DOCKER_HOST_FROM_CONTAINER
        environment['HOSTNAME'] = docker_host
        environment['LOCALSTACK_HOSTNAME'] = docker_host
        if context: