‘./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).