Added gitlab-ci.yml for automatic docker image compelation on gitlab.

.gitlab-ci.yml

+before_script: 
+    - 'docker login -u gitlab-ci-token -p $CI_BUILD_TOKEN $CI_REGISTRY'
+
+pdf_compiler:
+    image: docker:latest 
+    script:
+        - 'docker build --no-cache -t $CI_REGISTRY_IMAGE
+            --build-arg https_proxy="$HTTPS_PROXY"
+            --build-arg http_proxy="$HTTP_PROXY"
+            --build-arg no_proxy="$no_proxy" 
+            .'
+        - 'docker push $CI_REGISTRY_IMAGE'
+    environment:
+        name: production
+