Merge branch 'feature/gb/use-gitlabktl-to-build-serverless-applications' into 'master'

Use `gitlabktl` to build serverless applications

Closes #61184

See merge request gitlab-org/gitlab-ce!29258

changelogs/unreleased/feature-gb-use-gitlabktl-to-build-serverless-applications.yml

+---
+title: Use to 'gitlabktl' build serverless applications
+merge_request: 29258
+author:
+type: added

lib/gitlab/ci/templates/Serverless.gitlab-ci.yml

@@ -8,15 +8,9 @@ stages:
   - deploy
 
 .serverless:build:image:
-  variables:
-    DOCKERFILE: "Dockerfile"
   stage: build
-  image:
-    name: gcr.io/kaniko-project/executor:debug
-    entrypoint: [""]
-  script:
-    - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
-    - /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/$DOCKERFILE --destination $CI_REGISTRY_IMAGE
+  image: registry.gitlab.com/gitlab-org/gitlabktl:latest
+  script: /usr/bin/gitlabktl app build
 
 .serverless:deploy:image:
   stage: deploy