Upgrade pip before using it in the docker images

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

Short description

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.