Added a few new Docker containers

Just added 4 new Docker container images:

• 'https://registry.hub.docker.com/yesw2000/ml-base:centos7-python39'
• 'https://registry.hub.docker.com/yesw2000/ml-pyroot:centos7-python39'
• 'https://registry.hub.docker.com/yesw2000/ml-tensorflow-cpu:centos7-python39'
• 'https://registry.hub.docker.com/yesw2000/ml-tensorflow-gpu:centos7-python39'

Please help approve the merge request.

Shuwei