Fix build arg usage

.gitlab-ci.yml

@@ -11,7 +11,7 @@ services:
   before_script:
     - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
   script:
-    - docker build -t $CI_REGISTRY_IMAGE:$PYTHON_VERSION --build-arg=$PYTHON_VERSION .
+    - docker build -t $CI_REGISTRY_IMAGE:$PYTHON_VERSION --build-arg pythonVer=$PYTHON_VERSION .
   after_script:
     - docker push $CI_REGISTRY_IMAGE:$PYTHON_VERSION