Rebuild varnish-6.2.0 Centos7 rpm packages with the help of mock.

mockbuild.sh

yum -y install mock
adduser mockbuild
usermod -a -G mock mockbuild
su - mockbuild
mock -r epel-7-x86_64 --init

wget https://packagecloud.io/varnishcache/varnish62/el/7/SRPMS/varnish-6.2.0-1.el7.src.rpm

# rebuild without modification
mock -r epel-7-x86_64 --target=x86_64 \
  --define "srcname varnish-6.2.0" \
  --define "versiontag 6.2.0" \
  --define "releasetag 1" \
  --rebuild varnish-6.2.0-1.el7.src.rpm
  
  
# rebuild with some modification in spec or source
rpm -ivh varnish-6.2.0-1.el7.src.rpm
cd ~/rpmbuild/
vi SPECS/varnish.spec
mock -r epel-7-x86_64 --target=x86_64 \
   --define "srcname varnish-6.2.0" \
   --define "versiontag 6.2.0" \
   --define "releasetag 1" \
   --spec=SPECS/varnish.spec \
   --sources=SOURCES/ \
   --buildsrpm

cp /var/lib/mock/epel-7-x86_64/result/varnish-6.2.0-1.el7.src.rpm /tmp/

mock -r epel-7-x86_64 --target=x86_64 \
  --define "srcname varnish-6.2.0" \
  --define "versiontag 6.2.0" \
  --define "releasetag 1" \
  --rebuild /tmp/varnish-6.2.0-1.el7.src.rpm
  
rpm -qip /var/lib/mock/epel-7-x86_64/result/varnish-6.2.0-1.el7.x86_64.rpm
  

The varnish package is a nice example for a mock build since it fails on a plain Centos7 due to a python version clash.