fix: use plugin-kaniko for docs image build

The raw gcr.io/kaniko-project/executor with commands: doesn't work
properly in Woodpecker. Switch to woodpeckerci/plugin-kaniko with
settings: to match other component builds.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/adapter/templates/templates/skeleton/.woodpecker.yml.tmpl

@@ -124,15 +124,18 @@ steps:
 
   # Build and push docs-nginx image (skipped if no docs build output)
   build-docs-image:
-    image: gcr.io/kaniko-project/executor:latest
     depends_on: [build-docs]
-    commands:
-      - |
-        if [ ! -d "docs/build" ]; then
-          echo "==> No docs/build/ directory, skipping image build"
-          exit 0
-        fi
-      - /kaniko/executor --dockerfile=Dockerfile.nginx --context=docs --insecure --destination=registry.threesix.ai/{{PROJECT_NAME}}-docs:latest --destination=registry.threesix.ai/{{PROJECT_NAME}}-docs:${CI_COMMIT_SHA:0:8}
+    image: woodpeckerci/plugin-kaniko
+    settings:
+      registry: registry.threesix.ai
+      repo: {{PROJECT_NAME}}-docs
+      tags:
+        - latest
+        - ${CI_COMMIT_SHA:0:8}
+      context: docs
+      dockerfile: docs/Dockerfile.nginx
+      cache: true
+      skip-tls-verify: true
     when:
       branch: main
       event: push