For openSUSE Leap 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Documentation:Containers/openSUSE_Leap_15.5/Documentation:Containers.repo
zypper refresh
zypper install docbook_5
For openSUSE Leap 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Documentation:Containers/openSUSE_Leap_15.4/Documentation:Containers.repo
zypper refresh
zypper install docbook_5
For containers 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Documentation:Containers/containers_15.5/Documentation:Containers.repo
zypper refresh
zypper install docbook_5