fix: use buildx --platform linux/arm64 for ARM64 cross-build

.gitea/workflows/docker.yml

@@ -41,21 +41,20 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Docker info
+      - name: Build MCP Server (ARM64)
         run: |
-          docker info --format 'Arch: {{ .Architecture }}, OS: {{ .OSType }}'
-          echo "Building ARM64 natively — no buildx needed"
-
-      - name: Build MCP Server
-        run: |
-          docker build \
+          docker buildx build \
+            --platform linux/arm64 \
+            --load \
             --tag ${{ env.IMAGE_MCP }}:latest \
             --tag ${{ env.IMAGE_MCP }}:${{ github.sha }} \
             --file Dockerfile .
 
-      - name: Build Canvas Server
+      - name: Build Canvas Server (ARM64)
         run: |
-          docker build \
+          docker buildx build \
+            --platform linux/arm64 \
+            --load \
             --tag ${{ env.IMAGE_CANVAS }}:latest \
             --tag ${{ env.IMAGE_CANVAS }}:${{ github.sha }} \
             --file Dockerfile.canvas .