
base and builder are currently pinned to 3.7, which makes it hard to consider upgrading to python-3.8. To help with that, make python-3.7 and python-3.8 images, but point latest as 3.7 for the time being. Then add version-specific provides and FROM lines so that we can start being deliberate and clear about our version choice. Change-Id: Ibf1d846d5c4a005547785124567ce2900e272a7a