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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!
CaptureOutput, FuncThread, TMP_FILES, short_uid, save_file,
    to_str, run, cp_r, json_safe, get_free_tcp_port)
from localstack.services.install import INSTALL_PATH_LOCALSTACK_FAT_JAR
LAMBDA_EXECUTOR_JAR = INSTALL_PATH_LOCALSTACK_FAT_JAR
LAMBDA_EXECUTOR_CLASS = 'cloud.localstack.LambdaExecutor'
EVENT_FILE_PATTERN = '%s/lambda.event.*.json' % config.TMP_FOLDER
LAMBDA_RUNTIME_PYTHON27 = 'python2.7'
LAMBDA_RUNTIME_PYTHON36 = 'python3.6'
LAMBDA_RUNTIME_NODEJS = 'nodejs'
LAMBDA_RUNTIME_NODEJS610 = 'nodejs6.10'
LAMBDA_RUNTIME_NODEJS810 = 'nodejs8.10'
LAMBDA_RUNTIME_NODEJS10X = 'nodejs10.x'
LAMBDA_RUNTIME_JAVA8 = 'java8'
LAMBDA_RUNTIME_DOTNETCORE2 = 'dotnetcore2.0'
LAMBDA_RUNTIME_DOTNETCORE21 = 'dotnetcore2.1'
LAMBDA_RUNTIME_GOLANG = 'go1.x'
LAMBDA_RUNTIME_RUBY = 'ruby'
LAMBDA_RUNTIME_RUBY25 = 'ruby2.5'
LAMBDA_RUNTIME_CUSTOM_RUNTIME = 'provided'
LAMBDA_EVENT_FILE = 'event_file.json'
LAMBDA_SERVER_UNIQUE_PORTS = 500
LAMBDA_SERVER_PORT_OFFSET = 5000
LOG = logging.getLogger(__name__)
MAX_CONTAINER_IDLE_TIME_MS = 600 * 1000
class LambdaExecutor(object):
    """ Base class for Lambda executors. Subclasses must overwrite the _execute method """
    def __init__(self):
        
        self.function_invoke_times = {}
    def execute(self, func_arn, func_details, event, context=None, version=None, asynchronous=False):