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.