separate frontend and binaries' build into different stages, fix mkdir command

.gitlab-ci.yml

 stages:
-  - build
+  - build-frontend
+  - build-binaries
 
 build-frontend:
-  stage: build
+  stage: build-frontend
   image: node:latest
   tags:
     - docker
@@ -15,7 +16,7 @@ build-frontend:
       sass style.scss:style.css
   after_script:
     - |
-      mkdir dist/assets
+      mkdir -p dist/assets
       cp main.js dist/assets
       cp style.css dist/assets
       cp index.html dist/assets
@@ -24,12 +25,10 @@ build-frontend:
       - dist/
 
 build-binaries:
-  stage: build
+  stage: build-binaries
   image: ocamlpro/ocaml:4.12
   tags:
     - docker
-  dependencies:
-    - build-frontend
   before_script:
     - sudo apk add bash openssl-libs-static
   script: