ci: enable BuildKit (DOCKER_BUILDKIT=1) so Dockerfile cache mounts build

The legacy builder ignores '# syntax=docker/dockerfile:1.7' and fails at the
first 'RUN --mount=type=cache' in the deps stage, which is why runs #193, #382
and #433 all died in ~3s at Step 5/38. Activating dockerd's integrated BuildKit
backend fixes the build so :latest publishes again.

.gitea/workflows/docker-build.yml

@@ -25,6 +25,13 @@ jobs:
           password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Build and Push
+        env:
+          # This Dockerfile uses BuildKit features (`# syntax=docker/dockerfile:1.7`
+          # and `RUN --mount=type=cache`). Without this the CLI falls back to the
+          # legacy builder, which ignores the syntax directive and fails at the first
+          # `--mount`. DOCKER_BUILDKIT=1 activates dockerd's integrated BuildKit
+          # backend (no separate buildx plugin required).
+          DOCKER_BUILDKIT: "1"
         run: |
           IMAGE="registry.alwisp.com/${{ gitea.repository }}"
           GIT_SHA="$(git rev-parse --short HEAD)"