For your dockerized snap, there's no need to declare slots in your snap(consumer) if you try to use docker command shared by docker-snap(producer). The slots are needed to be declared on producer end.
For any bash where you invoke the docker or docker-compose, you need to append docker-executables and docker-cli in plugs field and connect them upon installation to get the privilege to run docker command within your snap sandbox.
Meanwhile, as we announced in this thread, we just added the content-interface support in latest version docker-snap and promoted them to candidate channel of the respective tracks.
- Latest/candidate: 17.03.2-ce-1
- 17.03/candidate: 17.03.2-ce-1
- 17.06/candidate: 17.06.0-ce-1
So you need to refresh your docker snap to one of those three if you install the snap from stable channel.
snap refresh docker --candidate
or install it from candidate channel from latest track directly.
snap install docker --candidate
I'd recommend taking a look at this thread to learn how to use content-interface in the snap, especially on the consumer end.
Also It would be better if you can upload your whole project somewhere github or launchpad, then I can take a closer look if you have any further question.