diff --git a/.forgejo/workflows/build-images.yml b/.forgejo/workflows/build-images.yml
index 7817c7e..b4cee0a 100644
--- a/.forgejo/workflows/build-images.yml
+++ b/.forgejo/workflows/build-images.yml
@@ -21,6 +21,11 @@ jobs:
     env:
       REGISTRY: harbor.basicstack.de
       IMAGE_REPO: harbor.basicstack.de/basicstack/signing-service
+      # The signing-service Dockerfile uses `RUN --mount=type=cache` which
+      # the legacy Docker builder rejects; enable BuildKit so the daemon
+      # routes the build through the Dockerfile frontend declared by
+      # `# syntax=docker/dockerfile:1.7`.
+      DOCKER_BUILDKIT: "1"
     steps:
       - name: Install git + buildx client + ca-certs
         run: |