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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!
SQS_PORT_EXTERNAL = int(os.environ.get('SQS_PORT_EXTERNAL') or 0)
LOCALSTACK_HOSTNAME = os.environ.get('LOCALSTACK_HOSTNAME', '').strip() or HOSTNAME
LAMBDA_REMOTE_DOCKER = os.environ.get('LAMBDA_REMOTE_DOCKER', '').lower().strip() in TRUE_VALUES
LAMBDA_DOCKER_NETWORK = os.environ.get('LAMBDA_DOCKER_NETWORK', '').strip()
LAMBDA_CONTAINER_REGISTRY = os.environ.get('LAMBDA_CONTAINER_REGISTRY', '').strip() or DEFAULT_LAMBDA_CONTAINER_REGISTRY
DATA_DIR = os.environ.get('DATA_DIR', '').strip()
TMP_FOLDER = os.path.join(tempfile.gettempdir(), 'localstack')
for folder in [DATA_DIR, TMP_FOLDER]:
    if folder and not os.path.exists(folder):
        try:
            os.makedirs(folder)
        except Exception:
            
            
            pass
if TMP_FOLDER.startswith('/var/folders/') and os.path.exists('/private%s' % TMP_FOLDER):
    TMP_FOLDER = '/private%s' % TMP_FOLDER
HOST_TMP_FOLDER = os.environ.get('HOST_TMP_FOLDER', TMP_FOLDER)
USE_SSL = os.environ.get('USE_SSL', '').strip() in TRUE_STRINGS
DEFAULT_ENCODING = 'utf-8'
DOCKER_SOCK = os.environ.get('DOCKER_SOCK', '').strip() or '/var/run/docker.sock'
DOCKER_FLAGS = os.environ.get('DOCKER_FLAGS', '').strip()
DOCKER_CMD = os.environ.get('DOCKER_CMD', '').strip() or 'docker'
START_WEB = os.environ.get('START_WEB', '').strip() not in FALSE_STRINGS
PORT_WEB_UI = int(os.environ.get('PORT_WEB_UI', '').strip() or DEFAULT_PORT_WEB_UI)
PORT_WEB_UI_SSL = PORT_WEB_UI + 1
DOCKER_BRIDGE_IP = os.environ.get('DOCKER_BRIDGE_IP', '').strip()
EXTRA_CORS_ALLOWED_HEADERS = os.environ.get('EXTRA_CORS_ALLOWED_HEADERS', '').strip()
EXTRA_CORS_EXPOSE_HEADERS = os.environ.get('EXTRA_CORS_EXPOSE_HEADERS', '').strip()
def has_docker():