ci(release): add backstage image job

[morey-tech]

• Add backstage job to build and push the image with the latest and first 8 of the commit sha.
  • Instead of directly using the docker build action, I use the yarn build-image script to keep the CI build consistent with the local dev build.
• Rename the release job to configuration (corresponds to the resulting image).
• Consolidate workflows into a central release.yaml workflow file.

[morey-tech]

I've preped the showcase-backstage package to allow actions write access.