Question

Can you use docker-compose files to start services in Docker 1.12 swarm-mode?

Answer 1

Yess, you can. you must download/install the version that includes the experimental features (check the github).

You essentially follow these instructions https://github.com/docker/docker/blob/master/experimental/docker-stacks-and-bundles.md
to go from the docker-compose.yml file to a distributed application bundle and then to an application stack (this is when your services are actually run).
$docker-compose bundle
$ docker deploy [OPTIONS] STACK