Update .gitlab-ci.yml file

.gitlab-ci.yml

 stages:
   - build
-  - test
+  - push
+
+variables:
+  IMAGE_NAME: registry.gitlab.fel.cvut.cz/your-username/test-app
+
 build:
   stage: build
   script:
-    - echo "hello"
-    - mkdir folder
-    - touch folder/test.txt
-  artifacts:
-    paths:
-      - folder
+    - docker build -t $IMAGE_NAME:${CI_COMMIT_SHORT_SHA} .
+  only:
+    - main
 
-test:
-  stage: test
+push:
+  stage: push
   script:
-    - echo "tests"
-    - test -f "folder/test.txt"
+    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
+    - docker push $IMAGE_NAME:${CI_COMMIT_SHORT_SHA}
+  only:
+    - main