Dockerfile: Update to newer base and Python version
Dockerfile: Update to newer base and Python version

- Update the base ubunutu image to 18.10 and switch to using Python3 for
installing WA.
- Fix typo in documenation.

ARM-software/workload-automation,setrofim/workload-automation,setrofim/workload-automation,ARM-software/workload-automation,setrofim/workload-automation,ARM-software/workload-automation,ARM-software/workload-automation,setrofim/workload-automation