drop docker/* gitea actions to avoid 2+ min git-clone overhead per run

Gitea's act runner re-clones every referenced GitHub action on every build
because it has no action cache. docker/setup-buildx-action alone was taking
~2 minutes to clone before the build even started.

buildx is already bundled in gitea/runner-images:ubuntu-latest, so call
'docker buildx build --push' directly with --cache-from/--cache-to pointing
at a registry buildcache tag. Keeps the layer caching benefit, skips the
action-clone tax entirely.

.gitea/workflows/build.yml

@@ -14,9 +14,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
-
       - name: Log in to Gitea Container Registry
         run: echo "${{ secrets.REGISTRY_TOKEN }}" | docker login "$REGISTRY" -u "${{ github.actor }}" --password-stdin
 
@@ -24,13 +21,15 @@ jobs:
         id: meta
         run: echo "version=$(jq -r .version package.json)" >> "$GITHUB_OUTPUT"
 
-      - name: Build and push
-        uses: docker/build-push-action@v6
-        with:
-          context: .
-          push: true
-          tags: |
-            ${{ env.REGISTRY }}/${{ env.IMAGE }}:latest
-            ${{ env.REGISTRY }}/${{ env.IMAGE }}:${{ steps.meta.outputs.version }}
-          cache-from: type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE }}:buildcache
-          cache-to: type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE }}:buildcache,mode=max
+      # buildx is already bundled in the gitea/runner-images ubuntu image — no need
+      # for docker/setup-buildx-action, which costs ~2 minutes of git-clone on every run.
+      - name: Build and push with registry cache
+        run: |
+          docker buildx create --use --name ci-builder --driver docker-container || docker buildx use ci-builder
+          docker buildx build \
+            --push \
+            --cache-from "type=registry,ref=$REGISTRY/$IMAGE:buildcache" \
+            --cache-to   "type=registry,ref=$REGISTRY/$IMAGE:buildcache,mode=max" \
+            -t "$REGISTRY/$IMAGE:latest" \
+            -t "$REGISTRY/$IMAGE:${{ steps.meta.outputs.version }}" \
+            .