Switch frontend build to kaniko to avoid ephemeral-storage eviction

plugins/docker uses Docker-in-Docker which consumes significant
ephemeral storage for image layers. On nodes with disk pressure
(k8s-node1), the build pod gets evicted before completing.

Switch to plugins/kaniko which builds OCI images without a Docker
daemon, using significantly less ephemeral storage. Enable kaniko's
built-in layer caching via a dedicated cache repo.

.drone.yml

@@ -22,12 +22,7 @@ steps:
       - npx vitest run
 
   - name: Build frontend image
-    image: plugins/docker
-    resources:
-      requests:
-        memory: 2147483648
-      limits:
-        memory: 4294967296
+    image: plugins/kaniko
     settings:
       username: viktorbarzin
       password:
@@ -35,9 +30,8 @@ steps:
       repo: viktorbarzin/immoweb
       dockerfile: frontend/Dockerfile
       context: frontend
-      cache_from:
-        - viktorbarzin/immoweb:builder
-        - viktorbarzin/immoweb:latest
+      enable_cache: true
+      cache_repo: viktorbarzin/immoweb-cache
       tags:
         - latest
         - ${DRONE_BUILD_NUMBER}