Service in extends option should be optional

[aanm]

I think it would be better if the service option to be optional.
compose would look through extends' file and find if there's a service with the same name of the service where the extends option is.

#docker-compose.yml
web:
  extends:
    file: common-services.yml
    #service: web
  environment:
    - DEBUG=1
  cpu_shares: 5

#common-services.yml
web:
  build: .
  ports:
    - "8000:8000"
  volumes:
    - "/data"

[dnephin]

I think that might be too magical for the extends feature. extends is designed to target a specific service, not assume that it's an override.

There is a new feature in master #2051 that works as a direct override (not targeting a specific service).

Does that work?

[aanm]

@dnephin Yes, I was looking for something similar to that.