Dockerfile: Use gitlab.cs.pub.ro registry

Replace default DockerHub registry.

Signed-off-by: Razvan Deaconescu <razvan.deaconescu@upb.ro>

Dockerfile

-FROM razvandeax/operating-systems-assignments-base
+FROM gitlab.cs.pub.ro:5050/operating-systems/assignments-docker-base
 
 COPY ./checker ${CHECKER_DATA_DIRECTORY}
 RUN mkdir ${CHECKER_DATA_DIRECTORY}/../tests