Add ability to build image using multiple runners

.woodpecker/docker.yaml

@@ -1,3 +1,10 @@
+matrix:
+  platform:
+    - linux/amd64
+    - linux/arm64
+
+labels:
+  platform: ${platform}
 
 when:
   branch: main
@@ -7,17 +14,31 @@ variables:
   - &repo source.hodakov.me/${CI_REPO_OWNER}/phpbb
 
 steps:
-  dryrun:
+  test-build-arm64:
     image: woodpeckerci/plugin-docker-buildx:5.1.0
     privileged: true
     settings:
       dockerfile: Dockerfile
-      platforms: linux/arm64/v8,linux/amd64
+      platforms: linux/arm64/v8
       dry_run: true
       repo: *repo
       auto_tag: true
     when:
       event: pull_request
+      platform: linux/arm64
+
+  test-build-amd64:
+    image: woodpeckerci/plugin-docker-buildx:5.1.0
+    privileged: true
+    settings:
+      dockerfile: Dockerfile
+      platforms: linux/amd64
+      dry_run: true
+      repo: *repo
+      auto_tag: true
+    when:
+      event: pull_request
+      platform: linux/amd64
 
   publish:
     image: woodpeckerci/plugin-docker-buildx:5.1.0