Change tag from "stable" to "alpha" on Docker Hub

[pdurbin]

I'm having second thoughts about using the term "stable" for Docker images: https://preview.guides.gdcc.io/en/develop/container/base-image.html#supported-image-tags

We want to communicate that these images are not intended for production use. Perhaps we could change "stable" to "alpha" (I'm open to suggestions.)

This should be a trivial change so I'm giving this a size of 3.

Tags appear at https://hub.docker.com/r/gdcc/base/tags