add GH workflow action to create the build image
add GH workflow action to create the build image

Based on https://github.com/fpco/stackage-server/blob/master/.github/workflows/base.yml

Image is pushed to docker.pkg.github.com.
Later we should switch from docker to current beta container registry ghcr.io

mrkkrp/stackage,simonmichael/stackage,iand675/stackage,Daniel-Diaz/stackage,jonschoning/stackage