used fixed version of images in GitLab CI

.gitlab-ci.yml

@@ -2,7 +2,7 @@ default:
   tags:
     - docker-host-linux
   image:
-    name: "cr.gitlab.switch.ch/helm-charts/shibboleth-sp/pipeline-images/helm:latest"
+    name: "cr.gitlab.switch.ch/helm-charts/shibboleth-sp/pipeline-images/helm:v3.6.0"
 
 variables:
   NAMESPACE: "shibboleth-sp-helm-chart"