Remain with CentOS 7 for GitLab CI
Quoting the commit message:
When GParted GitLab Continous Integration was setup, CentOS 7 image was used for a slow moving distribution and Ubuntu as a different, faster moving distribution.
CentOS 8 has recently been released . To avoid automatically switching to it when an official CentOS 8 docker image is made available , explicitly specifiy the CentOS 7 image.
 Release for CentOS Linux 8 and CentOS Streams (2019-09-24)
 Docker Official Images, The official build of CentOS