Merge branch 'docker-image-testing' into 'master'

Docker image testing

Let GitLab build and push the image to the repo's registry.

See merge request !1

.gitlab-ci.yml

+before_script:
+  - docker info
+
+stages:
+  - build
+
+build_image:
+  image: docker:20.10.3
+  stage: build
+  services:
+    - docker:20.10.3-dind
+  tags:
+    - Docker_5
+  script:
+    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
+    - if [ "$CI_COMMIT_TAG" == "" ] && [ "$CI_COMMIT_SHORT_SHA" != "" ]; then NAME="$CI_COMMIT_SHORT_SHA"; else NAME="$CI_COMMIT_TAG"; fi
+    - docker build -t $CI_REGISTRY/the_red_freak/jupyterhub-extended/jupyterhubextended:$NAME .
+    - docker push $CI_REGISTRY/the_red_freak/jupyterhub-extended/jupyterhubextended:$NAME