Add workflow to automate backstage builds

[omrishiv]

What this does

This takes the work from https://github.com/cnoe-io/backstage-image-builder and merges it into this repository with an added Workflow file to automate building, tagging, and pushing the backstage image to ghcr. It uses the format ghcr.io/cnoe-io/backstage-app:GITSHA as the image name. This image will be built on every commit into the backstage-app repo, whether on a push to main or a PR. It also writes the image attestation to the repository for added supply chain security.

Example

https://github.com/omrishiv/backstage-app/actions/runs/9067164285

[jessesanford]

This is great. Thank you. @nabuskey I think you should probably take a look at this though.

[omrishiv]

@nabuskey Do you mind: https://stackoverflow.com/a/72585915

[OneCricketeer]

Is there an example of building this image with Argo Workflows and pushing into Gitea OCI registry? This would aid with faster development, I think. Ref #20

[nabuskey]

@OneCricketeer we don't have that yet. BUT we are building an example for building and pushing to the internal gitea registry as part of a workshop we are creating. It's still in WIP but is coming soon :)

If you can't wait you can probably base it off of this: https://github.com/cnoe-io/backstage-app/blob/main/.github/workflows/build-and-push.yaml

[OneCricketeer]

@nabuskey I believe I am part of that workshop 😉