Update .gitlab-ci.yml

.gitlab-ci.yml

+stages:
+  - build
+
+build_image:
+  stage: build
+  image: docker:18.09.7
+  only:
+    - master
+  services:
+    - 'docker:18.09.7-dind'
+  script:
+    - docker build -t clarinpl/tag-converter  .
+    - echo $DOCKER_PASSWORD > pass.txt
+    - cat pass.txt | docker login --username $DOCKER_USERNAME --password-stdin
+    - rm pass.txt
+    - docker push clarinpl/tag-converter