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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!
"""
        Return the Java classpath, using the parent folder of the
        given archive as the base folder.
        The result contains any *.jar files in the base folder, as
        well as any JAR files in the "lib/*" subfolder living
        alongside the supplied java archive (.jar or .zip).
        :param archive: an absolute path to a .jar or .zip Java archive
        :return: the Java classpath, relative to the base dir of "archive"
        """
        entries = ['.']
        base_dir = os.path.dirname(archive)
        for pattern in ['%s/*.jar', '%s/lib/*.jar']:
            for entry in glob.glob(pattern % base_dir):
                if os.path.realpath(archive) != os.path.realpath(entry):
                    entries.append(os.path.relpath(entry, base_dir))
        
        
        entries.append(os.path.relpath(archive, base_dir))
        result = ':'.join(entries)
        return result
EXECUTOR_LOCAL = LambdaExecutorLocal()
EXECUTOR_CONTAINERS_SEPARATE = LambdaExecutorSeparateContainers()
EXECUTOR_CONTAINERS_REUSE = LambdaExecutorReuseContainers()
DEFAULT_EXECUTOR = EXECUTOR_LOCAL
AVAILABLE_EXECUTORS = {
    'local': EXECUTOR_LOCAL,
    'docker': EXECUTOR_CONTAINERS_SEPARATE,
    'docker-reuse': EXECUTOR_CONTAINERS_REUSE
}