Provide image digest as an output

[DukeOfKirkcaldy]

The image digest is a required input for steps that follow the build process, such as actions/attest-build-provenance or for adding the built Docker image to JFrog Build Info.

Currently, the image digest is provided by the [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.