Document and implement jupyterhub image tagging

[LaVLaS]

In attempt to standardize the image upgrade process, we need to document and implement a process and steps for tagging images.

I'm proposing that all JupyterHub images stick to a tagging format of {jupyterhub image base}-{unique number} where

• jupyterhub image base is the base builder image. Currently we are using the 3.0.7 image from jupyterhub-quickstart
• unique-number is any unique indentifier for this image build. It can be just a simple integer or the short hash of the commit this image was built from.

Based on the current base image 3.0.7 and repo commit of 952741b the new tag would be jupyterhub:3.0.7-952741b

[LaVLaS]

Closing this since this is more appropriate to track and document this in the Open Data Hub deployment of jupyterhub.