add staging deployment to CI

.gitlab-ci.yml

@@ -211,7 +211,7 @@ deploy-back-staging:
       when: always
   variables:
     DOMAIN: eatfast.test.cs-campus.fr
-    PRIVATE_KEY: "$SSH_PRIVATE_KEY"
+    PRIVATE_KEY: "$SSH_PRIVATE_KEY_STAGING"
     IMAGE: "back"
   dependencies: []
 
@@ -225,7 +225,7 @@ deploy-front-staging:
       when: always
   variables:
     DOMAIN: eatfast.test.cs-campus.fr
-    PRIVATE_KEY: "$SSH_PRIVATE_KEY"
+    PRIVATE_KEY: "$SSH_PRIVATE_KEY_STAGING"
     IMAGE: "front"
   dependencies:
     - install-npm-packages