Provide image digest as an output

The image digest is a required input for steps that follow the build process, such as [actions/attest-build-provenance](https://github.com/actions/attest-build-provenance) or for [adding the built Docker image to JFrog Build Info](https://github.com/jfrog/jfrog-github-example/blob/50ad3b6bd875427e68095afb0d95bc56b5f79e07/.github/workflows/build-publish.yml#L112).

Currently, the image digest is provided by the [[docker/build-push-action](https://github.com/docker/build-push-action)](https://github.com/docker/build-push-action), but it is not available in the `devcontainers/ci` action. It would be useful to add the image digest as an output to support these use cases and ensure smooth integration when migrating existing workflows.

Inizia leggendo l’implementazione della GitHub Action devcontainers/ci e i suoi output esistenti, quindi confronta il modo in cui docker/build-push-action espone un digest dell’immagine. Aggiungi il digest come output dell’action e verifica che uno step successivo del workflow possa utilizzarlo per la provenienza o l’integrazione con JFrog Build Info.

Scritto dal modello di indicizzazione a partire dal testo della issue.