ci: install docker buildx plugin; switch to buildx build --load

DOCKER_BUILDKIT=1 requires the buildx plugin which isn't installed on
the runner. Install it explicitly then use docker buildx build --load
(outputs to local daemon so the push step can tag+push normally).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/build-push.yaml

@@ -66,15 +66,22 @@ jobs:
           echo "primary=${REGISTRY}:${ENV_TAG}" >> "$GITHUB_OUTPUT"
           echo "sha=${SHA}" >> "$GITHUB_OUTPUT"
 
+      - name: Install docker buildx
+        run: |
+          mkdir -p ~/.docker/cli-plugins
+          BUILDX_URL="https://github.com/docker/buildx/releases/download/v0.20.1/buildx-v0.20.1.linux-amd64"
+          curl -fsSL "$BUILDX_URL" -o ~/.docker/cli-plugins/docker-buildx
+          chmod +x ~/.docker/cli-plugins/docker-buildx
+          docker buildx version
+
       - name: Build Gitea image
-        env:
-          DOCKER_BUILDKIT: "1"
         run: |
           TAGS=""
           for t in ${{ steps.tag.outputs.tags }}; do
             TAGS="$TAGS -t $t"
           done
-          docker build \
+          docker buildx build \
+            --load \
             --build-arg GITEA_VERSION="neuron-$(git rev-parse --short HEAD)" \
             --build-arg TAGS="sqlite sqlite_unlock_notify" \
             $TAGS \