ci: attach provenance and SBOM attestations to the published image

[kobihikri]

Hi, and thanks for obscura.

.github/workflows/docker.yml publishes the image, but the pushed manifest carries no provenance or SBOM attestation. Someone pulling it cannot check that it was built by this workflow, from this repository, at that tag.

A privacy-oriented tool is a natural fit for verifiable provenance: the reason to self-host it is not wanting to trust someone else's instance, and the same reasoning applies to the image.

The change is two lines on the build step:

          push: ...
          provenance: mode=max
          sbom: true

BuildKit attaches both to the image manifest, so they travel with the image. No permissions change is needed — nothing has to gain id-token, and your tag and cache configuration are untouched.

docker buildx imagetools inspect <image>:<tag> --format '{{ json .Provenance }}'

Two caveats worth stating: mode=max records the full build including build arguments, so provenance: true is the smaller option if any have ever been sensitive; and attestations add an extra manifest to the index, which the registry supports.

No SLSA level claimed — the attestation is what BuildKit produces.

Disclosure: I used AI assistance to help spot this and prepare the change, and I read the workflow myself.

[kobihikri]

Correction — I got a fact wrong in this PR, and I would rather flag it myself than let it sit.

I wrote that the pushed manifest "carries no provenance or SBOM attestation". That is half wrong, and the wrong half matters.

Provenance is already there. For public repositories, docker/build-push-action adds provenance attestations with mode=max by default — Docker's documentation states it plainly: "Public repos: provenance attestations with mode=max are automatically added". I checked published images and they do already carry attestation manifests. So the provenance: mode=max line in my diff makes existing behaviour explicit; it does not add anything new.

The SBOM is genuinely new. That part stands — the same page says "SBOM attestations aren't automatically added to the image", and sbom: true is what enables them.

I also wrote in the caveats that provenance: true gives "a smaller record". That is wrong as well: true resolves to max on a public repo, and the smaller setting is provenance: mode=min.

So the honest description of this PR is: it adds an SBOM attestation, and pins the provenance mode explicitly instead of relying on the default. Both are still defensible — an explicit line means the behaviour will not change quietly if the default ever does — but it is a smaller change than my description implied, and you should judge it on that basis rather than on what I originally wrote.

Happy to retitle and rewrite the description accordingly, or to close this if the SBOM alone is not worth the diff to you. Either is fine — just say which and I will act on it.

Apologies for the inaccuracy. It was caught by a maintainer reviewing the same change on another project, and they were right to.