[x] bug report
[ ] feature request

# Detailed description
`lambda_executor.py` current retrieves the container entrypoint from the docker image via `docker inspect --format="{{ .ContainerConfig.Entrypoint }}" ..`. This is fragile and may be missing depending on how the image in question is built. There is a `config` block _and_ a `containerconfig` block that are mostly the same, but sometimes different depending what built and what version of that thing built the image, for example we are seeing the entrypoint missing on images built with Docker for Mac 2.5.0.1, but not on earlier versions, others using `podman` are noticing the fragility in other projects:

https://github.com/containers/podman/issues/2017

## Expected behavior
entrypoint value is picked up from a validly built container

## Actual behavior
entrypoint is sometimes an empty string, which then for a `provided` lambda executor ends up with a script error trying to execute the handler name.

The simple fix is to change `--format="{{ .ContainerConfig.Entrypoint }}"` to `--format="{{ .Config.Entrypoint }}"` which seems like the more canonical way of getting that value.



┆Issue is synchronized with this [Jira Task](https://localstack.atlassian.net/browse/LOC-54) by [Unito](https://www.unito.io/learn-more)

self.cleanup()
        
        if config.LAMBDA_REMOVE_CONTAINERS:
            self.start_idle_container_destroyer_interval()
    def cleanup(self, arn=None):
        if arn:
            self.function_invoke_times.pop(arn, None)
            return self.destroy_docker_container(arn)
        self.function_invoke_times = {}
        return self.destroy_existing_docker_containers()
    def prime_docker_container(self, runtime, func_arn, env_vars, lambda_cwd):
        """
        Prepares a persistent docker container for a specific function.
        :param runtime: Lamda runtime environment. python2.7, nodejs6.10, etc.
        :param func_arn: The ARN of the lambda function.
        :param env_vars: The environment variables for the lambda.
        :param lambda_cwd: The local directory containing the code for the lambda function.
        :return: ContainerInfo class containing the container name and default entry point.
        """
        with self.docker_container_lock:
            
            container_name = self.get_container_name(func_arn)
            docker_cmd = self._docker_cmd()
            status = self.get_docker_container_status(func_arn)
            LOG.debug('Priming docker container (status "%s"): %s' % (status, container_name))