ci: refactor CI stages

Signed-off-by: Klagarge <remi@heredero.ch>

gateway/.gitlab-ci.yml

@@ -5,12 +5,12 @@ default:
   image: $DOCKER_IMAGE
 
 stages:
-  - build-docker
+  - gateway-build
 
 # This job runs only when Dockerfile changes
-docker-build:
+gateway-docker-build:
   image: docker:latest
-  stage: build-docker
+  stage: gateway-build
   services:
     - docker:dind
   script: