## troubleshooting --fakeroot issues

    $ singularity build --fakeroot ...
    FATAL:   could not use fakeroot: no user entry found for {USER}

This probably means that _subuid_ range is not defined in the "/etc/subuid"
file. On up-to-date system subuid/subgid ranges are automatically assigned
when a new user is added (e.g. `adduser {USER}`) but on systems upgraded
from prior Debian releases {USER} created in old environment before upgrade
may not have _subuid_ mapping.

"usermod" command have "--add-subuids" and "--add-subgids" options but it
does not check "/etc/login.defs" for ranges.
An awkward solution may be to add a new temporary user, apply her ranges to
{USER} (in "/etc/subuid" and in "/etc/subgid") then remove a temporary user
(e.g. `deluser --remove-home {USER}`).

The following command show the subuids and subgids of the current user:

    cat  /etc/s*id|grep $USER

---

    $ singularity build --fakeroot ...
    ERROR  : Failed to create container namespaces

The above error means that the system has been booted without
"kernel.unprivileged_userns_clone=1" option.
To enable user namespaces use the following command:

    sudo sysctl kernel.unprivileged_userns_clone=1


See also

  https://sylabs.io/guides/3.4/user-guide/fakeroot.html
  https://github.com/sylabs/singularity/issues/4361

    singularity config fakeroot --help
