Change uid and gid of user nobody in CentOS 8 image

Dockerfile_c8

@@ -56,4 +56,8 @@ RUN yes | xrdsssadmin -k eos-test del /etc/eos.keytab; \
 # Change owner of /var/spool/xrootd directory to daemon
 RUN chown daemon:daemon /var/spool/xrootd
 
+# The system tests have a strong bias towards nobody having uid=99 guid=99
+RUN usermod -u 99 nobody; \
+    groupmod -g 99 nobody
+
 ENTRYPOINT ["/bin/bash"]