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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!
md5, new_tmp_file, parse_chunked_data, is_number, now_utc, safe_requests, isoformat_milliseconds)
from localstack.utils.aws import aws_stack, aws_responses
from localstack.utils.analytics import event_publisher
from localstack.utils.aws.aws_models import LambdaFunction
from localstack.utils.cloudwatch.cloudwatch_util import cloudwatched
APP_NAME = 'lambda_api'
PATH_ROOT = '/2015-03-31'
ARCHIVE_FILE_PATTERN = '%s/lambda.handler.*.jar' % config.TMP_FOLDER
LAMBDA_SCRIPT_PATTERN = '%s/lambda_script_*.py' % config.TMP_FOLDER
LAMBDA_RUNTIMES = [LAMBDA_RUNTIME_PYTHON27, LAMBDA_RUNTIME_PYTHON36,
    LAMBDA_RUNTIME_DOTNETCORE2, LAMBDA_RUNTIME_DOTNETCORE21, LAMBDA_RUNTIME_NODEJS,
    LAMBDA_RUNTIME_NODEJS610, LAMBDA_RUNTIME_NODEJS810, LAMBDA_RUNTIME_JAVA8, LAMBDA_RUNTIME_RUBY,
    LAMBDA_RUNTIME_RUBY25]
LAMBDA_DEFAULT_TIMEOUT = 3
LAMBDA_DEFAULT_HANDLER = 'handler.handler'
LAMBDA_DEFAULT_RUNTIME = LAMBDA_RUNTIME_PYTHON27
LAMBDA_DEFAULT_STARTING_POSITION = 'LATEST'
LAMBDA_ZIP_FILE_NAME = 'original_lambda_archive.zip'
LAMBDA_JAR_FILE_NAME = 'original_lambda_archive.jar'
app = Flask(APP_NAME)
arn_to_lambda = {}
event_source_mappings = []
LOG = logging.getLogger(__name__)
exec_mutex = threading.Semaphore(1)
DO_USE_DOCKER = None
LAMBDA_EXECUTOR = lambda_executors.AVAILABLE_EXECUTORS.get(config.LAMBDA_EXECUTOR, lambda_executors.DEFAULT_EXECUTOR)
IAM_POLICY_VERSION = '2012-10-17'
POLICY_NAME_PATTERN = 'lambda_policy_%s'
BUCKET_MARKER_LOCAL = '__local__'
class ClientError(Exception):