feat: enable multiplatform Docker image builds for amd64 and arm64

Add QEMU setup and platforms configuration to build Docker images
for both linux/amd64 and linux/arm64 architectures.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

.gitea/workflows/docker-build.yml

@@ -16,6 +16,9 @@ jobs:
           username: ${{ secrets.USERNAME }}
           password: ${{ secrets.ACCESS_TOKEN }}
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
@@ -23,4 +26,5 @@ jobs:
         uses: docker/build-push-action@v6
         with:
           push: true
+          platforms: linux/amd64,linux/arm64
           tags: "g.eliaskohout.de/eliaskohout/axolotl-server:${{gitea.ref_name}},g.eliaskohout.de/eliaskohout/axolotl-server:latest"