'./docker-compose.yml', service must be a mapping, not a NoneType

YAML is indentation dependent. This

services:zoo1:

is a mapping with two keys that both map to null (i.e. None in Python).

Whereas:

services:   zoo1:

is a mapping nested in a mapping. The key 'services' has as value a mapping with key zoo1 (which again has a value null).

version: '2'                                                                    services:                                                                            autodiscovery:                                                                     build: ./autodiscovery/                                                            mem_limit: 128m                                                                    expose:                                                                              - 53                                                                               - 8300                                                                             - 8301                                                                             - 8302                                                                             - 8400                                                                             - 8500                                                                           ports:                                                                               - 8500:8500                                                                      dns:                                                                                 - 127.0.0.1  

just put one space. before.