docker-compose gives ERROR: Cannot locate specified Dockerfile: Dockerfile

Here is how to specify dockerfile. Sorry I cannot do it in comments as formatting is not supported over there.

https://docs.docker.com/compose/compose-file/#context

app:
    build:
      context: app
      dockerfile: Dockerfile

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).