Defines runner methods for ChefSpec matchers

This enables us to target these resources by using ```ruby chef_run.docker_image('resource-name')```.

Example:

```ruby
  it 'notifies the database container to redeploy' do
    image = chef_run.docker_image('database')
    expect(image).to notify('docker_container[database]').to(:redeploy).immediately
  end
```