Rootless Podman auto adjust the permissions when using `--userns=keep` on volumes.

podman-volume-auto-chown-files.sh

#!/usr/bin/env bash

# This demonstrates that podman (rootless)
# `--userns=keep-id` will auto set permissions of mounted volumes to the user.

set -eu -o pipefail

if podman info | yq ".host.security.rootless" | grep -q "true"; then
    echo "--> Your are running ROOTLESS podman."
else
    echo "--> Your are running ROOTFUL podman."
fi

podman build --no-cache -t "test:latest" -f <(
    cat <<EOF
FROM ubuntu:latest
# Create user and group ci
RUN useradd -d /home/ci -U ci
RUN echo "Groups:" && cat /etc/group | tail -1
RUN echo "Passwd:" && cat /etc/passwd | tail -1
EOF
)

echo
echo "--> Create root files."
podman run -it -v test-vol:/work \
    -w /work test:latest bash -c "echo 'Bingo it works!' > A && chmod 700 A && ls -aln A"

echo
echo "--> List files as user 'ci' does not work when not run with keep-id."
podman run -it \
    --user ci \
    -v test-vol:/work \
    -w /work test:latest bash -c 'echo User: $(id) && ls -aln A && cat A' || true

echo
echo "--> List files as user 'ci' with 'keep-id'."
podman run -it --userns=keep-id:uid=1001,gid=1001 \
    --user ci \
    -v test-vol:/work \
    -w /work test:latest bash -c 'echo User: $(id) && ls -aln A && cat A'