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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!
opts = config.LAMBDA_JAVA_OPTS or ''
        if '_debug_port_' in opts:
            if not cls.debug_java_port:
                cls.debug_java_port = get_free_tcp_port()
            opts = opts.replace('_debug_port_', ('%s' % cls.debug_java_port))
        return opts
    @classmethod
    def get_host_path_for_path_in_docker(cls, path):
        return re.sub(r'^%s/(.*)$' % config.TMP_FOLDER,
                      r'%s/\1' % config.HOST_TMP_FOLDER, path)
    @classmethod
    def docker_image_for_runtime(cls, runtime):
        docker_tag = runtime
        docker_image = config.LAMBDA_CONTAINER_REGISTRY
        
        
        lambdas_to_add_prefix = ['dotnetcore', 'python']
        if docker_image == 'lambci/lambda' and any(img in docker_tag for img in lambdas_to_add_prefix):
            docker_tag = '20191117-%s' % docker_tag
        return '"%s:%s"' % (docker_image, docker_tag)
    @classmethod
    def get_java_classpath(cls, archive):