[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