Specify Docker image for shared runners

.gitlab-ci.yml

+image: maven:latest
+
 stages:
     - compile
     - test
@@ -16,4 +18,4 @@ test:
 deploy:
     stage: deploy
     script:
-      - dpl --provider=heroku --app=jlp19-simplewebapp --api-key=$Heroku_API_Key
\ No newline at end of file
+      - dpl --provider=heroku --app=jlp19-simplewebapp --api-key=$Heroku_API_Key