For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ojkastl_buildservice:Branch_Virtualization_containers/openSUSE_Tumbleweed/home:ojkastl_buildservice:Branch_Virtualization_containers.repo
zypper refresh
zypper install apko
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ojkastl_buildservice:Branch_Virtualization_containers/15.6/home:ojkastl_buildservice:Branch_Virtualization_containers.repo
zypper refresh
zypper install apko
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ojkastl_buildservice:Branch_Virtualization_containers/15.5/home:ojkastl_buildservice:Branch_Virtualization_containers.repo
zypper refresh
zypper install apko