Update .gitlab-ci.yml file

.gitlab-ci.yml

-image: docker:19.03.0
-
-variables: 
-  DOCKER_DRIVER: overlay2
-  DOCKER_TLS_CERTDIR: "/certs" 
-
-services:
-  - docker:19.03.0-dind
-
-before_script:
-  - docker info
-
 job:
     script:
+      - apt update
       - echo "Hello world"
       - docker build .
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