Maybe ?

.gitlab-ci.yml

@@ -1,7 +1,7 @@
 image: node
 
 before_script:
-    - cd frontend && yarn install
+    - yarn --cwd frontend install
 
 cache:
     paths:
@@ -9,7 +9,7 @@ cache:
 
 pages:
     script:
-        - cd frontend && yarn run generate
+        - yarn --cwd frontend run generate
     artifacts:
         paths:
             - public

frontend/nuxt.config.js

@@ -115,6 +115,11 @@ export default {
             },
         },
     },
+
+    generate: {
+        dir: '../public',
+    },
+
     /*
      ** Build configuration
      */