run Chromium as user

[umumax]

I'm having issue with running Chromium as user in RootlessKit's namespace. Or should I say nested namespace, because I'm creating netns within it.
Chromium doesn't like to be run as root. It requires sandboxing disabled to run as root.

Below are commands I'm running to set everything up:

./rootlesskit --copy-up=/etc --copy-up=/run  --net=slirp4netns --disable-host-loopback --state-dir=${XDG_RUNTIME_DIR}/rootlesskit bash

/usr/sbin/openvpn --writepid "/var/run/openvpn-netns-$$.pid" --config /home/user/fr.conf &

ip netns add vpn
ip netns exec vpn ip link set dev lo up
ip link set dev "$1" up netns vpn mtu "$2"
ip netns exec vpn ip addr add dev "$1" "$4/24" ${ifconfig_broadcast:+broadcast "$ifconfig_broadcast"}
route_vpn_gateway=$(ip netns exec vpn ip route list table main | awk -v tun="$1" '/tun/ { print $9}')
ip netns exec vpn ip route add default via "$route_vpn_gateway"
ip netns exec vpn ip link set lo up

Chromium's error is not informative but firefox's one gives some clues:

ip netns exec vpn /sbin/runuser -u user -- chromium
[27842:27842:0608/065854.467030:ERROR:(-1)] Check failed: false.
Trace/breakpoint trap

ip netns exec vpn /sbin/runuser -u user -- firefox
(Firefox-esr:27927): dconf-CRITICAL **: 07:00:17.176: unable to create directory '/run/user/1000/dconf': Permission denied.  dconf will not work properly.

Could someone let me know what should be done to make this work?

Thank you