Add ability to build image using multiple runners (!1)

Reviewed-on: #1

.woodpecker/docker.yaml

@@ -1,3 +1,10 @@
+matrix:
+  platform:
+    - linux/amd64
+    - linux/arm64
+
+labels:
+  platform: ${platform}
 
 when:
   branch: main
@@ -7,12 +14,12 @@ variables:
   - &repo source.hodakov.me/${CI_REPO_OWNER}/phpbb
 
 steps:
-  dryrun:
+  test-build:
     image: woodpeckerci/plugin-docker-buildx:5.1.0
     privileged: true
     settings:
       dockerfile: Dockerfile
-      platforms: linux/arm64/v8,linux/amd64
+      platforms: ${platform}
       dry_run: true
       repo: *repo
       auto_tag: true
@@ -24,7 +31,7 @@ steps:
     privileged: true
     settings:
       dockerfile: Dockerfile
-      platforms: linux/arm64/v8,linux/amd64
+      platforms: ${platform}
       repo: *repo
       registry: source.hodakov.me
       auto_tag: true