fix(ci): pin TRIVY_PLATFORM per matrix arch (post-#477 follow-up)

PR #477 moved Trivy from the merge-* job into the per-arch build-*
matrix scanning by digest. The amd64 leg works; the arm64 leg
crashes with:

  remote error: no child with platform linux/amd64 in index
  ghcr.io/.../<image>@sha256:<digest>

Root cause: docker/build-push-action wraps every push in an OCI
index — the actual image manifest sits next to a SLSA provenance
attestation manifest as siblings under the digest. Trivy's remote
backend defaults to linux/amd64 when resolving an index, so:

  - amd64 leg → looks for amd64 child → finds the amd64 image → ok.
  - arm64 leg → looks for amd64 child → finds NO amd64 child
    (the only platform child is arm64) → fails.

Fix: set TRIVY_PLATFORM = ${{ matrix.platform }} on each leg's
Trivy step. Each scanner then asks for its own arch and finds it.
SLSA provenance attestation stays attached to the per-arch images
— a real win for supply-chain visibility we'd lose if we'd
disabled provenance instead.

amd64 was the only thing keeping CI partly green; this restores
full green across both legs without touching the build artifact
shape.

.github/workflows/docker-build.yml

@@ -173,6 +173,17 @@ jobs:
     - name: Run Trivy vulnerability scanner (per-arch, by digest)
       if: steps.push-decision.outputs.push == 'true'
       uses: aquasecurity/trivy-action@v0.36.0
+      env:
+        # docker/build-push-action wraps every push in an OCI index
+        # (carries the SLSA provenance attestation alongside the
+        # actual image). Trivy's remote backend defaults to
+        # linux/amd64 regardless of host arch when resolving an
+        # index, which makes the arm64 leg crash with "no child
+        # with platform linux/amd64". Telling Trivy which child to
+        # scan keeps the provenance attestation intact and fixes
+        # the resolver crash. Pin to matrix.platform so each leg
+        # scans its own arch.
+        TRIVY_PLATFORM: ${{ matrix.platform }}
       with:
         image-ref: ${{ env.REGISTRY }}/${{ env.BACKEND_IMAGE_NAME }}@${{ steps.build.outputs.digest }}
         format: 'sarif'
@@ -379,6 +390,12 @@ jobs:
     - name: Run Trivy vulnerability scanner (per-arch, by digest)
       if: steps.push-decision.outputs.push == 'true'
       uses: aquasecurity/trivy-action@v0.36.0
+      env:
+        # See build-backend for the rationale — pin Trivy's platform
+        # to the matrix arch so its remote-index resolver picks the
+        # right child instead of defaulting to linux/amd64 and
+        # crashing on the arm64 leg.
+        TRIVY_PLATFORM: ${{ matrix.platform }}
       with:
         image-ref: ${{ env.REGISTRY }}/${{ env.FRONTEND_IMAGE_NAME }}@${{ steps.build.outputs.digest }}
         format: 'sarif'