Unable to automatically set up `/etc/subuid` and `/etc/subgid` for new user

Created originally in Butane by @mogeko: https://github.com/coreos/butane/issues/494

I'm using Butane to deploy a Fedora CoreOS.

I want to create a non-privileged user (nonroot) specifically to run the Podman container.

But it seems that Butane did not set /etc/subuid and /etc/subgid for him:

variant: fcos
version: 1.5.0

passwd:
  groups:
    - name: nonroot
      gid: 65532
  users:
    - name: mogeko
      gecos: Mogeko
      ssh_authorized_keys:
        - ssh-rsa ...
      groups:
        - wheel
        - sudo
    - name: nonroot
      gecos: Unprivileged User
      uid: 65532
      primary_group: nonroot
      no_log_init: true
      shell: /sbin/nologin

# ...

$ id
uid=1001(mogeko) gid=1001(mogeko) groups=1001(mogeko),10(wheel),16(sudo) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

$ sudo --user=nonroot id
uid=65532(nonroot) gid=65532(nonroot) groups=65532(nonroot) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

$ cat /etc/subuid
core:524288:65536
mogeko:589824:65536

$ cat /etc/subgid
core:524288:65536
mogeko:589824:65536

Start at the passwd.users configuration shown in the issue and trace how Ignition provisions new users on Fedora CoreOS. Check how /etc/subuid and /etc/subgid are currently handled for created users, then verify that the requested nonroot user receives appropriate entries and that existing behavior remains intact.