Update .gitlab-ci.yml file to force the two docker images to be run...

Update .gitlab-ci.yml file to force the two docker images to be run sequentially, in order to avoid race condition and file lock errors

.gitlab-ci.yml

@@ -8,11 +8,14 @@
 #
 # This template uses one generic job with conditional builds
 # for the default branch and all other (MR) branches.
+stages:
+  - build_jupyterlab
+  - build_jupyterhub
 
 docker-build_jupyterlab:
   # Use the official docker image.
   image: docker:latest
-  stage: build
+  stage: build_jupyterlab
   services:
     - docker:dind
   before_script:
@@ -32,7 +35,7 @@ docker-build_jupyterlab:
 docker-build_jupyterhub:
   # Use the official docker image.
   image: docker:latest
-  stage: build
+  stage: build_jupyterhub
   services:
     - docker:dind
   before_script: