DockerBot now classified

I would be tempted to put the responsibility for container_id to the parent class (because the parent class is already using it). But that is just an idea.

```python
class DockerInstance(object):
    def __init__(self, container_id):
        self.container_id = container_id

class SeleniumDocker(DockerInstance):
    def __init__(self, bindings, image):
        # whatever
        super(SeleniumDocker, self).__init__(_dgci(sel_create_info, "id"))
```