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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!
event = {'timestamp': int(log_time), 'message': line}
            log_events.append(event)
        if not log_events:
            return
        logs_client.put_log_events(
            logGroupName=log_group_name,
            logStreamName=log_stream_name,
            logEvents=log_events
        )
    def run_lambda_executor(self, cmd, event=None, env_vars={}):
        process = run(cmd, asynchronous=True, stderr=subprocess.PIPE, outfile=subprocess.PIPE, env_vars=env_vars,
                      stdin=True)
        result, log_output = process.communicate(input=event)
        try:
            result = to_str(result).strip()
        except Exception:
            pass
        log_output = to_str(log_output).strip()
        return_code = process.returncode
        
        
        
        if isinstance(result, six.string_types) and '\n' in result:
            additional_logs, _, result = result.rpartition('\n')
            log_output += '\n%s' % additional_logs
        if return_code != 0:
            raise Exception('Lambda process returned error status code: %s. Result: %s. Output:\n%s' %
                (return_code, result, log_output))
        return result, log_output
class ContainerInfo: