fix: kaniko standalone missing included variables

Fix for #16

```
ERROR: Job failed: failed to pull image "gcr.io/kaniko-project/executor:-
debug" with specified policies [always]: invalid reference format
(manager.go:250:0s)

```

This happens when the kaniko template is used without including
`templates/standard.yml`.

kaniko.yml

 # Note: this is not a full template, just the basis
+
+include:
+  - local: .gitlab-ci-other-versions.yml
+
 .kaniko_base:
   image:
     name: gcr.io/kaniko-project/executor:${GL_COMMON_CI_TASKS_KANIKO_VERSION}-debug