cicd: adjust busybox entrypoint for Docker executor

No idea why it doesn't accept /busybox/sh.

.gitlab-ci.yml

 build:
   image:
     name: gcr.io/kaniko-project/executor:debug
-    entrypoint: ["/busybox/sh"]
+    entrypoint: [""]
   script:
     - |
         for i in "$CI_PROJECT_DIR"/*/Dockerfile; do