Build multi-platform image for linux/amd64 and linux/arm64

The builder stage cross-compiles Caddy for the target platform, so no
emulation is needed. The module check runs against the amd64 image; PR
checks also build arm64.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitea/workflows/pr-checks.yml

@@ -16,8 +16,16 @@ jobs:
 
       - uses: actions/checkout@v4
 
+      - name: Set up Docker buildx
+        run: |
+          docker buildx rm multi 2>/dev/null || true
+          docker buildx create --name multi --driver docker-container --use
+
       - name: Build
-        run: docker build --pull --tag caddy-desec-docker:check .
+        run: docker buildx build --pull --platform linux/amd64 --tag caddy-desec-docker:check --load .
+
+      - name: Build arm64
+        run: docker buildx build --pull --platform linux/arm64 .
 
       - name: Check the deSEC module is built in
         run: docker run --rm caddy-desec-docker:check caddy list-modules | grep -x dns.providers.desec