Make DATASET variable editable in CI

.gitlab-ci.yml

@@ -6,13 +6,9 @@ default:
       - node_modules/
 
 variables:
-  ## This variable can be overridden at pipeline execution time.
-  ## Possible values:
-  ## - dole
-  ## - jorf
-  ## - kali
-  ## - legi
-  DATASET: undefined
+  DATASET:
+    description: "Dataset to import (either dole, jorf, kali or legi"
+    value: dole
   ## Ensure that all the Git tags are retrieved.
   ## https://docs.gitlab.com/ee/user/project/pipelines/settings.html#git-shallow-clone
   GIT_DEPTH: 0