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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 is_s3_copy_request(self, headers, path):
        return 'x-amz-copy-source' in headers or 'x-amz-copy-source' in path
    def forward_request(self, method, path, data, headers):
        
        parsed_path = urlparse.urlparse(path)
        
        
        if 's3.amazonaws.com' not in headers.get('host', ''):
            headers['host'] = 'localhost'
        
        if 'Content-MD5' in headers and not self.is_s3_copy_request(headers, path):
            response = check_content_md5(data, headers)
            if response is not None:
                return response
        modified_data = None
        
        bucket_name = get_bucket_name(path, headers)
        if method == 'PUT' and not re.match(BUCKET_NAME_REGEX, bucket_name):
            if len(parsed_path.path) <= 1:
                return error_response('Unable to extract valid bucket name. Please ensure that your AWS SDK is ' +
                    'configured to use path style addressing, or send a valid <Bucket>.s3.amazonaws.com "Host" header',
                    'InvalidBucketName', status_code=400)
            return error_response('The specified bucket is not valid.', 'InvalidBucketName', status_code=400)
        
        to_find = to_bytes('<LocationConstraint>us-east-1</LocationConstraint>')
        if data and data.startswith(to_bytes('<')) and to_find in data:
            modified_data = data.replace(to_find, to_bytes(''))