Add support for 'docker image inspect' in our docker abstraction.

Docker's command line tool for docker inspect can take either a container, an image, or a task as its argument, and return a JSON array containing low-level information about that container, image or task.

However, the current docker inspect support in our docker abstraction only supports inspecting containers (not images or tasks). We should expand this to (at least) support images.

In particular, this additional functionality is motivated by the upcoming GPU support, which needs to inspect the labels in a docker image to decide if it should inject the required Nvidia volumes into a container.