Switch Docker image to use Python 3
Switch Docker image to use Python 3

There's oddly no `python36-pip` package available and it seemed a bit sad to
install Python 3.4, so pip is instead installed via the `ensurepip` module, as
suggested here:
https://stackoverflow.com/a/52518512/878291

powdahound/ec2instances.info,powdahound/ec2instances.info,powdahound/ec2instances.info