Update .gitlab-ci.yml to build with no cache

.gitlab-ci.yml

@@ -11,7 +11,7 @@ build_image:
    - shell
   stage: build
   script:
-    - docker build -t registry.cybbh.space/courseware-as-code/courseware-builder/container .
+    - docker build --no-cache -t registry.cybbh.space/courseware-as-code/courseware-builder/container .
   only:
     - master