ci: use crane to mirror base images, then build ai-assistant

- First step: crane copy base images from Docker Hub to registry.nxtgauge.com
  - rust:alpine, rust:1.87-alpine, alpine:3.20, node:20-alpine
- Second step: kaniko build ai-assistant image
- No DinD required - crane handles registry-to-registry copy

.woodpecker.yml

@@ -3,13 +3,28 @@ when:
   event: push
 
 steps:
-  - name: build-and-push
+  - name: mirror-base-images
+    image: gcr.io/go-containerregistry/crane:debug
+    environment:
+      REG_USER:
+        from_secret: REGISTRY_USERNAME
+      REG_PASS:
+        from_secret: REGISTRY_PASSWORD
+    commands:
+      - crane auth login registry.nxtgauge.com -u "$REG_USER" -p "$REG_PASS"
+      - crane copy docker.io/library/rust:alpine registry.nxtgauge.com/rust:alpine
+      - crane copy docker.io/library/rust:1.87-alpine registry.nxtgauge.com/rust:1.87-alpine
+      - crane copy docker.io/library/alpine:3.20 registry.nxtgauge.com/alpine:3.20
+      - crane copy docker.io/library/node:20-alpine registry.nxtgauge.com/node:20-alpine
+
+  - name: build-and-push-ai-assistant
     image: woodpeckerci/plugin-kaniko:2.1.1
     settings:
       registry: registry.nxtgauge.com
       repo: nxtgauge-ai-assistant
       dockerfile: Dockerfile
       tags:
+        - ${CI_COMMIT_SHA}
         - latest
         - high-performance-latest
       username: