Update autograder/Dockerfile, .gitlab-ci.yml files

.gitlab-ci.yml

@@ -22,5 +22,5 @@ docker-build-autograder:
   before_script:
     - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
   script:
-    - docker build --pull -t "$CI_REGISTRY_IMAGE:autograder" Dockerfile_autograder
+    - docker build --pull -t "$CI_REGISTRY_IMAGE:autograder" autograder
     - docker push "$CI_REGISTRY_IMAGE:autograder"