You need to build your own image. The official resource is here but here is a simple Dockerfile:
FROM registry.hub.docker.com/gentoo/portage:latest as portage
FROM registry.hub.docker.com/gentoo/stage3:systemd
COPY --from=portage /var/db/repos/gentoo /var/db/repos/gentoo
Build it using either podman or docker:
podman build . -t gentoo-distrobox
or
docker build . -t gentoo-distrobox
and it's ready to be used:
distrobox create --image localhost/gentoo-distrobox:latest