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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!
def __init__(self, msg, code=400):
        super(ClientError, self).__init__(msg)
        self.code = code
        self.msg = msg
    def get_response(self):
        if isinstance(self.msg, Response):
            return self.msg
        return error_response(self.msg, self.code)
class LambdaContext(object):
    def __init__(self, func_details, qualifier=None):
        self.function_name = func_details.name()
        self.function_version = func_details.get_qualifier_version(qualifier)
        self.invoked_function_arn = func_details.arn()
        if qualifier:
            self.invoked_function_arn += ':' + qualifier
    def get_remaining_time_in_millis(self):
        
        return 1000 * 60
def cleanup():
    global event_source_mappings, arn_to_lambda
    arn_to_lambda = {}
    event_source_mappings = []
    LAMBDA_EXECUTOR.cleanup()
def func_arn(function_name):
    return aws_stack.lambda_function_arn(function_name)
def add_function_mapping(lambda_name, lambda_handler, lambda_cwd=None):
    arn = func_arn(lambda_name)
    arn_to_lambda[arn].versions.get('$LATEST')['Function'] = lambda_handler
    arn_to_lambda[arn].cwd = lambda_cwd
def add_event_source(function_name, source_arn, enabled):