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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!
legacy_patterns = re.compile(r'^(.+)\.s3\.?(-external-1)?\.amazonaws\.com(\.[a-z]+)?$')
    
    
    for pattern in [common_pattern, dualstack_pattern, legacy_patterns]:
        match = pattern.match(host)
        if match:
            bucket_name = match.groups()[0]
            break
    
    
    return normalize_bucket_name(bucket_name)
def handle_notification_request(bucket, method, data):
    response = Response()
    response.status_code = 200
    response._content = ''
    if method == 'GET':
        
        result = '<NotificationConfiguration xmlns="%s">' % XMLNS_S3
        if bucket in S3_NOTIFICATIONS:
            notifs = S3_NOTIFICATIONS[bucket]
            for notif in notifs:
                for dest in NOTIFICATION_DESTINATION_TYPES:
                    if dest in notif:
                        dest_dict = {
                            '%sConfiguration' % dest: {
                                'Id': uuid.uuid4(),
                                dest: notif[dest],
                                'Event': notif['Event'],
                                'Filter': notif['Filter']
                            }