ci: stop docker/build-push-action from uploading .dockerbuild artifacts

build-push-action v6 uploads a build-record artifact on every run with
no expiry; 768 of them had silently exhausted the account-wide Actions
artifact-storage quota, which then failed artifact uploads in every
other repo (qemu-lcgamboa's libqemu release CI among them). Disable the
upload via DOCKER_BUILD_RECORD_UPLOAD=false — the provenance records
were never consumed.

.github/workflows/docker-publish.yml

@@ -50,6 +50,12 @@ jobs:
 
       - name: Build and push Docker image
         uses: docker/build-push-action@v6
+        env:
+          # v6 uploads a .dockerbuild provenance artifact per build with NO
+          # expiry — 768 of them silently exhausted the account's Actions
+          # artifact-storage quota (which then broke artifact uploads in
+          # every other repo, e.g. qemu-lcgamboa's libqemu release CI).
+          DOCKER_BUILD_RECORD_UPLOAD: "false"
         with:
           context: .
           file: Dockerfile.standalone