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.