This pull request adds a new workflow to build and deploy Spack Docker containers
from GitHub Actions. In comparison with our current system where we use Dockerhub's
CI to build our Docker containers, this workflow will allow us to now build for multiple
architectures and deploy to multiple registries. (At the moment x86_64 and Arm64 because
ppc64le is throwing an error within archspec.)
As currently set up, the PR will build all of the current containers (minus Centos6 because
those yum repositories are no longer available?) as both x86_64 and Arm64 variants. The
workflow is currently setup to build and deploy containers nightly from develop as well as
on tagged releases. The workflow will also build, but NOT deploy containers on a pull request
for the purposes of testing this PR. At the moment it is setup to deploy the built containers to
GitHub's Container Registry although, support for also uploading to Dockerhub/Quay can be
included easily if we decide to keep releasing on Dockerhub/want to begin releasing on Quay.
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