v3.2.0

feat: add sbom input and digest output Erik Osterman (Cloud Posse) (@osterman) (#110)

## Summary - Add an `sbom` input (mirrors the existing `provenance` input) and pass it through to `docker/build-push-action@v7` to enable SBOM attestation generation. - Add a `digest` output sourced directly from the build step's own `digest` output, so downstream cosign signing steps don't need to `fromJSON()`-parse the `metadata` output. - Regenerate `README.md` (`atmos readme`) to document both the new input and output.

Test plan

• YAML validity checked locally (yaml.safe_load)
• CI workflows (test-docker-build*.yml) pass on this branch