Add docker build to CI

.gitlab-ci.yml

 stages:
+  - build_docker_image
   - test
   - quality-assurance
 
+docker:
+  stage: build_docker_image
+  script:
+    - docker login -u $CI_REGISTRY_USERNAME -p $CI_BUILD_TOKEN
+    - docker build -t $CI_REGISTRY_IMAGE:$CI_BUILD_REF_NAME .
+    - docker push $CI_REGISTRY_IMAGE:$CI_BUILD_REF_NAME
+  only:
+    - master
+    - beta
+
 # Debian Buster
 py37-django22:
   stage: test