[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)

return re.sub(r'^%s/(.*)$' % config.TMP_FOLDER,
                      r'%s/\1' % config.HOST_TMP_FOLDER, path)
    @classmethod
    def format_windows_path(cls, path):
        temp = path.replace(':', '').replace('\\', '/')
        if len(temp) >= 1 and temp[:1] != '/':
            temp = '/' + temp
        temp = '%s%s' % (config.WINDOWS_DOCKER_MOUNT_PREFIX, temp)
        return temp
    @classmethod
    def docker_image_for_runtime(cls, runtime):
        docker_tag = runtime
        docker_image = config.LAMBDA_CONTAINER_REGISTRY
        
        
        lambdas_to_add_prefix = ['dotnetcore2.0', 'dotnetcore2.1', 'python2.7', 'python3.6', 'python3.7']
        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_docker_remove_flag(cls):
        return '--rm' if config.LAMBDA_REMOVE_CONTAINERS else ''
    @classmethod
    def get_java_classpath(cls, archive):
        """
        Return the Java classpath, using the parent folder of the