Update .gitlab-ci.yml file

.gitlab-ci.yml

 build_image:
   script:
-    - docker buildah -t gitlab-registry.cern.ch/chweber/zx_postprocessingplotter:zx_limit_container .
+    - buildah build -t gitlab-registry.cern.ch/chweber/zx_postprocessingplotter:zx_limit_container .
     #- docker run my-docker-image /script/to/run/tests
\ No newline at end of file