Enable BuildKit for docker build in CI

The host runner's docker build was falling back to the legacy builder,
which can't parse the BuildKit-only RUN --mount=type=cache syntax in the
Dockerfile. Set DOCKER_BUILDKIT=1 to route the build through the daemon's
integrated BuildKit engine.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/docker-build.yml

@@ -21,6 +21,8 @@ jobs:
           password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Build and Push
+        env:
+          DOCKER_BUILDKIT: "1"
         run: |
           # gitea.repository is already "owner/repo" (e.g. jason/breedr).
           IMAGE="registry.alwisp.com/${{ gitea.repository }}"