buildah: max_user_namespaces not enable warn when set up environment not start with user namespace

Description

when run buildah inside container, it shows warning of enable max_user_namespace

buildah bud -t buildah-compile .
Error during unshare(CLONE_NEWUSER): Invalid argument
User namespaces are not enabled in /proc/sys/user/max_user_namespaces.
ERRO error parsing PID "": strconv.Atoi: parsing "": invalid syntax 
ERRO (unable to determine exit status) 

It seems like I should enable user namespace using command like echo 15000 > /proc/sys/user/max_user_namespaces. But what make me confused is that the Dockerfile of the image quay.io/buildah/stable shows it already set up env _BUILDAH_STARTED_IN_USERNS="" to stop start buildah with user namespace.

1. if it’s the problem of that I didn’t enable user namespace, why the env is not work?
2. if it’s not the problem of user namespace, how can I debug its root cause?

Steps to reproduce the issue:

podman run -it --rm quay.io/buildah/stable bash
cd
echo -e "FROM quay.io/buildah/stable\nRUN dnf -y update;dnf -y install java-1.8.0-openjdk-devel nodejs maven;dnf -y clean all" > Dockerfile
buildah bud -t buildah-compile .
Error during unshare(CLONE_NEWUSER): Invalid argument
User namespaces are not enabled in /proc/sys/user/max_user_namespaces.
ERRO error parsing PID "": strconv.Atoi: parsing "": invalid syntax 
ERRO (unable to determine exit status)

Describe the results you expected: buildah should work

Output of rpm -q buildah or apt list buildah:

# host
buildah-1.11.6-11.el7_8.x86_64

# container
buildah-1.14.8-1.fc32.x86_64

Output of buildah version:

# host
buildah version 1.11.6 (image-spec 1.0.1-dev, runtime-spec 1.0.1-dev)

# container
buildah version 1.14.8 (image-spec 1.0.1-dev, runtime-spec 1.0.1-dev)

Output of cat /etc/*release:

# host
CentOS Linux release 7.8.2003 (Core)

# container
Fedora release 32 (Thirty Two)

Output of uname -a:

# host
Linux localhost.localdomain 3.10.0-1062.4.1.el7.x86_64 #1 SMP Fri Oct 18 17:15:30 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

# container
Linux cdd19f290235 3.10.0-1062.4.1.el7.x86_64 #1 SMP Fri Oct 18 17:15:30 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

Output of cat /etc/containers/storage.conf:

[storage]
driver = "overlay"
runroot = "/var/run/containers/storage"
graphroot = "/var/lib/containers/storage"
[storage.options]
additionalimagestores = [
"/var/lib/shared",
]
[storage.options.overlay]
mount_program = "/usr/bin/fuse-overlayfs"
mountopt = "nodev,fsync=0"
[storage.options.thinpool]