Update .gitlab-ci.yml file

.gitlab-ci.yml

-image: docker:19.03.12
-
-variables:
-  DOCKER_HOST: tcp://docker:2375
-  DOCKER_TLS_CERTDIR: ""
-
-services:
-  - docker:19.03.12-dind
-
-before_script:
-  - docker info
+image: docker:stable
 
 build:
   stage: build
   script:
-    - docker build -t my-docker-image .
-    - docker run my-docker-image /script/to/run/tests
\ No newline at end of file
+    - docker-compose build
\ No newline at end of file