[WIP] Add docker resource

[SimonBaeumer]

Adds the docker resource to goss.

First draft:

docker:
  "alpine:3.7": # Image, Title or Name as title? 
    exist: true
    running: true
    image: "alpine:3.7"
    name: "alpine_name"
    count: 1

Fixes #57

Definition of done:

• [ ] Add documentation
• [ ] Define api / resource definition
• [ ] Implement add command
• [ ] Implement auto add
• [ ] Implement validation
• [ ] Add integration tests
• [ ] Add unit tests