Add attestations to CI pipeline

[marcus67]

Add attestations for both images to the pipeline, see snippets below

  # Attestation only works for public repositories
  - name: Generate artifact attestation
    uses: actions/attest-build-provenance@v1
    with:
      subject-name: accso/secure-check-plus
      subject-digest: ${{ steps.push.outputs.digest }}
      push-to-registry: true

  # Attestation only works for public repositories
  - name: Generate artifact attestation
    uses: actions/attest-build-provenance@v1
    with:
      subject-name: accso/secure-check-plus
      subject-digest: ${{ steps.push.outputs.digest }}
      push-to-registry: true