Add .gitlab-ci.yml

.gitlab-ci.yml

+image: docker:latest
+services:
+    - docker:dind
+    
+variables:
+    IMAGE_TAG: $DOCKER_USERNAME/gargantools:$CI_COMMIT_REF_SLUG
+    
+stages:
+    - build
+    
+build:
+    stage: build
+    script:
+        - echo $DOCKER_PASSWORD | docker login -u $DOCKER_USERNAME --password-stdin
+        - docker build -t $IMAGE_TAG .
+        - docker push $IMAGE_TAG
+        
+only:
+    - master
+        
\ No newline at end of file