It is a common pattern to define a base service which you want several services to extend, but it's currently very verbose to do this:

docker-compose-base.yml

base:
  environment:
    FOO: bar

docker-compose.yml

web:
  extends:
    file: docker-compose-base.yml
    service: base
  ...
db:
  extends:
    file: docker-compose-base.yml
    service: base
  ...

It should be possible to extend an incomplete service in a single compose file. Something like this:

base:
  abstract: true
  environment:
    FOO: bar
web:
  extends:
    service: base
  ...
db:
  extends:
    service: base

Design decisions:

• How should a service be marked as a service that shouldn't be run? (I've used "abstract" in the example above for the sake of illustration, pinched from Django's "abstract" models.)