Clean up the CI script

.gitlab-ci.yml

 image: docker:latest
 
-variables:
-  REPO: tech/soft/thesis/builder
-
 build:
   stage: build
   services:
     - docker:dind
   script:
     - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
-    - docker build -t $CI_REGISTRY/$REPO:latest .
-    - docker push $CI_REGISTRY/$REPO:latest
+    - docker build -t $CI_REGISTRY_IMAGE:latest .
+    - docker push $CI_REGISTRY_IMAGE:latest