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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!
response.status_code = 204
            return response
        
        if method == 'GET' and response.status_code == 404 and parsed.query != 'website':
            s3_client = aws_stack.connect_to_service('s3')
            try:
                
                s3_client.head_bucket(Bucket=bucket_name)
                website_config = s3_client.get_bucket_website(Bucket=bucket_name)
                error_doc_key = website_config.get('ErrorDocument', {}).get('Key')
                if error_doc_key:
                    error_object = s3_client.get_object(Bucket=bucket_name, Key=error_doc_key)
                    response.status_code = 200
                    response._content = error_object['Body'].read()
                    response.headers['content-length'] = len(response._content)
            except ClientError:
                
                pass
        if response:
            reset_content_length = False
            
            append_cors_headers(bucket_name, request_method=method, request_headers=headers, response=response)
            append_last_modified_headers(response=response)
            append_list_objects_marker(method, path, data, response)
            
            
            if method == 'PUT' and ('X-Amz-Security-Token=' in path or
                    'X-Amz-Credential=' in path or 'AWSAccessKeyId=' in path):
                response._content = ''
                reset_content_length = True