<!-- Love localstack? Please consider supporting our collective:
👉  https://opencollective.com/localstack/donate -->

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!
global SERVICE_PORTS, CONFIG_ENV_VARS
    SERVICE_PORTS = service_ports or parse_service_ports()
    globs = globals()
    
    for key, value in six.iteritems(DEFAULT_SERVICE_PORTS):
        key_upper = key.upper().replace('-', '_')
        
        port_var_name = 'PORT_%s' % key_upper
        port_number = service_port(key)
        globs[port_var_name] = port_number
        url = 'http%s://%s:%s' % ('s' if USE_SSL else '', LOCALSTACK_HOSTNAME, port_number)
        
        url_key = 'TEST_%s_URL' % key_upper
        globs[url_key] = url
        
        os.environ[url_key] = url
    
    os.environ['LOCALSTACK_HOSTNAME'] = LOCALSTACK_HOSTNAME
    
    CONFIG_ENV_VARS += ['LOCALSTACK_' + v for v in CONFIG_ENV_VARS if not v.startswith('LOCALSTACK_')]
    CONFIG_ENV_VARS = list(set(CONFIG_ENV_VARS))
def service_port(service_key):