fix(ci): add setup-buildx-action to fix build-image attestation failure

build-image has been failing on every run since serve GDI as a
validation service + buildable image (#27) landed, with:

  ERROR: failed to build: Attestation is not supported for the docker driver.
  Learn more at https://docs.docker.com/go/attestations/

docker/build-push-action@v6 requests provenance and SBOM attestations
(provenance: true, sbom: true), which require a `docker-container`
buildx builder. Without an explicit docker/setup-buildx-action step,
buildx falls back to the plain `docker` driver bound to the host
daemon's default context, which cannot emit attestations.

Add docker/setup-buildx-action@v3 before the build step to create a
docker-container builder (its default behavior), enabling attestations
to work as configured. Kept provenance/sbom enabled rather than
disabling them, since this image's sha tag is consumed and pinned by
prophet-platform deploy manifests.

Verified locally: a docker-container-driver buildx builder completes
`docker buildx build --attest type=provenance --attest
type=sbom,disabled=false ...` successfully (exports an attestation
manifest), confirming the driver change is sufficient to resolve this
exact error.

Author: mdheller

.github/workflows/build-image.yml

@@ -16,6 +16,12 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+        # Creates a docker-container builder (the default for this action).
+        # Required because provenance/SBOM attestations below are not
+        # supported by the plain `docker` driver you get without this step:
+        # "Attestation is not supported for the docker driver."
       - name: Log in to GHCR
         uses: docker/login-action@v3
         with: