Upgrade pip before using it in the docker images

Ghost User
2018-07-25T14:45:18+02:00

Upgrade to the latest version of `pip` before using it in the `Dockerfile`.

This fixes warnings like this while building the images:
```
You are using pip version 9.0.2, however version 10.0.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
```