Switch CI from plugin-docker-buildx to plugin-kaniko

docker-buildx requires privileged mode which is blocked in
Woodpecker v3. Kaniko builds in userspace, matching bcard setup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.woodpecker/build.yml

@@ -1,13 +1,13 @@
 when:
-  branch: main
-  event: push
+  - branch: main
+    event: push
 
 steps:
   - name: build-backend
-    image: woodpeckerci/plugin-docker-buildx
+    image: woodpeckerci/plugin-kaniko
     settings:
-      repo: git.mikhailkilin.ru/killingdruid/bbb-backend
       registry: git.mikhailkilin.ru
+      repo: killingdruid/bbb-backend
       dockerfile: backend/Dockerfile
       context: backend
       tags: latest
@@ -17,10 +17,10 @@ steps:
         from_secret: gitea_password
 
   - name: build-frontend
-    image: woodpeckerci/plugin-docker-buildx
+    image: woodpeckerci/plugin-kaniko
     settings:
-      repo: git.mikhailkilin.ru/killingdruid/bbb-frontend
       registry: git.mikhailkilin.ru
+      repo: killingdruid/bbb-frontend
       dockerfile: frontend/Dockerfile
       context: frontend
       tags: latest