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/build.yml

@@ -21,11 +21,16 @@ 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 "$IMAGE:latest" .
+        run: docker buildx build --pull --platform linux/amd64 --tag "$IMAGE:check" --load .
 
       - name: Check the deSEC module is built in
-        run: docker run --rm "$IMAGE:latest" caddy list-modules | grep -x dns.providers.desec
+        run: docker run --rm "$IMAGE:check" caddy list-modules | grep -x dns.providers.desec
 
       - name: Log in to the container registry
         env:
@@ -40,5 +45,10 @@ jobs:
             echo "No existing :latest to promote; skipping."
           fi
 
-      - name: Push "latest"
-        run: docker push "$IMAGE:latest"
+      - name: Build and push multi-arch "latest"
+        run: |
+          docker buildx build \
+            --platform linux/amd64,linux/arm64 \
+            --tag "$IMAGE:latest" \
+            --push \
+            .