[WIP] Add docker resource

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

SimonBaeumer / goss

[WIP] Add docker resource #63