handle no default attestations env var

[crazy-max]

fixes #1339

We should not set provenance attestation if BUILDX_NO_DEFAULT_ATTESTATIONS env var is set.

[tonistiigi]

Is the issue that env is not propagated to buildx or does this need custom handling? If former then maybe a better fix would be to just make sure all BUILDX_* env are forwarded.

[crazy-max]

or does this need custom handling

Yes it needs custom handling because the action is setting the provenance based on github events:

build-push-action/src/context.ts

Lines 248 to 260 in c566248

	} else if (!hasAttestProvenance && (await toolkit.buildkit.versionSatisfies(inputs.builder, '>=0.11.0')) && !Build.hasDockerExporter(inputs.outputs, inputs.load)) {
	// if provenance not specified in provenance or attests inputs and BuildKit
	// version compatible for attestation, set default provenance. Also needs
	// to make sure user doesn't want to explicitly load the image to docker.
	if (GitHub.context.payload.repository?.private ?? false) {
	// if this is a private repository, we set the default provenance
	// attributes being set in buildx: https://github.com/docker/buildx/blob/fb27e3f919dcbf614d7126b10c2bc2d0b1927eb6/build/build.go#L603
	args.push('--attest', `type=provenance,${Build.resolveProvenanceAttrs(`mode=min,inline-only=true`)}`);
	} else {
	// for a public repository, we set max provenance mode.
	args.push('--attest', `type=provenance,${Build.resolveProvenanceAttrs(`mode=max`)}`);
	}
	}