[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)

""" Contains basic information about a docker container. """
    def __init__(self, name, entry_point):
        self.name = name
        self.entry_point = entry_point
class LambdaExecutorContainers(LambdaExecutor):
    """ Abstract executor class for executing Lambda functions in Docker containers """
    def prepare_execution(self, func_arn, env_vars, runtime, command, handler, lambda_cwd):
        raise Exception('Not implemented')
    def _docker_cmd(self):
        """ Return the string to be used for running Docker commands. """
        return config.DOCKER_CMD
    def prepare_event(self, environment, event_body):
        """ Return the event as a stdin string. """
        
        environment['AWS_LAMBDA_EVENT_BODY'] = event_body
        return None
    def _execute(self, func_arn, func_details, event, context=None, version=None):
        lambda_cwd = func_details.cwd
        runtime = func_details.runtime
        handler = func_details.handler
        environment = self._prepare_environment(func_details)
        
        if config.USE_SSL:
            environment['USE_SSL'] = '1'
        
        if not event:
            LOG.warning('Empty event body specified for invocation of Lambda "%s"' % func_arn)
            event = {}
        event_body = json.dumps(json_safe(event))
        stdin = self.prepare_event(environment, event_body)