For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ojkastl_buildservice:containers:dumb-init/15.6/home:ojkastl_buildservice:containers:dumb-init.repo
zypper refresh
zypper install dumb-init
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ojkastl_buildservice:containers:dumb-init/15.5/home:ojkastl_buildservice:containers:dumb-init.repo
zypper refresh
zypper install dumb-init
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ojkastl_buildservice:containers:dumb-init/15.4/home:ojkastl_buildservice:containers:dumb-init.repo
zypper refresh
zypper install dumb-init