Added docker image build

.gitlab-ci.yml

@@ -1,5 +1,6 @@
 stages:
   - run
+  - build
 
 run_flask_app:
   stage: run
@@ -10,3 +11,13 @@ run_flask_app:
     - python app.py &
     - sleep 5
     - curl http://127.0.0.1:5000
+
+build_docker_image:
+  stage: build
+  image: docker:24
+  services:
+    - docker:24-dind
+  variables:
+    DOCKER_TLS_CERTDIR: "/certs"
+  script:
+    - docker build -t flask-app .