ci(images): enable BuildKit for signing-service image build

The signing-service Dockerfile ([DEV-609](/DEV/issues/DEV-609))
starts with `# syntax=docker/dockerfile:1.7` and uses
`RUN --mount=type=cache,...` for Go build/module caches. The legacy
Docker builder rejects both; the image-build job has been failing on
main since DEV-609 landed:

    the --mount option requires BuildKit. Refer to
    https://docs.docker.com/go/buildkit/ to learn how to build images
    with BuildKit enabled

Fix: set `DOCKER_BUILDKIT=1` on the job env so the daemon routes the
build through the declared Dockerfile frontend.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.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: |