fix: allow multi-platform builds using --push flag (#29)

plugin/impl.go

@@ -83,6 +83,10 @@ func (p *Plugin) Validate() error {
 	p.settings.Build.Ref = p.pipeline.Commit.Ref
 	p.settings.Daemon.Registry = p.settings.Login.Registry
 
+	if len(p.settings.Build.Platforms.Value()) > 1 && p.settings.Dryrun {
+		return fmt.Errorf("dryrun is not supported on multi-platform builds")
+	}
+
 	if p.settings.Build.TagsAuto {
 		// return true if tag event or default branch
 		if UseDefaultTag(