Improve multi-arch builds

- Use docker buildx to build the container for different architectures
- Upload build artifacts properly

.woodpecker.yml

@@ -1,10 +1,11 @@
 pipeline:
 
   docker:
-    image: plugins/docker
+    image: woodpeckerci/plugin-docker-buildx
     settings:
       registry: gitea.massivebox.net
       repo: gitea.massivebox.net/massivebox/ecodash
+      platforms: linux/amd64,linux/arm64
       auto_tag: true
       username: massivebox
       password:
@@ -30,7 +31,9 @@ pipeline:
     image: plugins/gitea-release
     settings:
       base_url: https://gitea.massivebox.net
-      files: ecodash-x86.zip ecodash-arm.zip
+      files: 
+        - ecodash-x86.zip
+        - ecodash-arm.zip
       api_key:
         from_secret: auth_token
       title: ${CI_COMMIT_TAG}