Move to the v1.12 API

The 1.12 API now can be told to always clean up intermediate containers when building an image, which is really useful for the plugin. The 1.12 API got introduced with Docker 1.0, so it's fairly stable.
It shouldn't be a big issue for the plugin to require a minimum of docker 1.0.
The 1.12 API changed the output of container inspections a little, but I don't feel like staying backwards compatible with sub-1.12 APIs (before Docker 1.0).