Avoid different outcomes every time we run "docker build":
- Use centos 7.8 base image
- Use fixed versions of CentOS & EPEL repos
- Pin mock & its dependencies to 1.4.16
- Don't install unnecessary packages
- Don't install unnecessary/conflicting repo files
Change-Id: Ic9a52f373809ed66a0e61675f4d6276fd62e6fa0
Signed-off-by: Davlet Panech <davlet.panech@windriver.com>