Add dryrun (#3)

Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://codeberg.org/woodpecker-plugins/plugin-docker-buildx/pulls/3

.woodpecker/main.yml

@@ -1,4 +1,14 @@
 pipeline:
+  dryrun:
+    image: plugins/kaniko
+    settings:
+      repo: woodpeckerci/plugin-docker-buildx
+      tags: latest
+      dockerfile: Dockerfile
+      no_push: true
+    when:
+      event: pull_request
+
   publish:
     image: plugins/kaniko
     settings:
@@ -8,4 +18,7 @@ pipeline:
       username:
         from_secret: docker_username
       password:
-        from_secret: docker_password
+        from_secret: docker_password
+    when:
+      event: [push, tag]
+      branch: main