clear before_script for docker building stages

build/ci/.build-container.yml

@@ -8,6 +8,7 @@ variables:
 
 build:docker:
   before_script:
+    - echo "override global before script"
   image: docker:19.03.12
   tags:
     - dind
@@ -25,6 +26,7 @@ build:docker:
 
 .deploy: &deploy
   before_script:
+    - echo "override global before script"
   image: docker:19.03.12
   stage: deploy
   needs: ["build:docker"]