Run as root:
setcap cap_net_raw+ep /bin/ping
getcap /bin/ping
should return:
/bin/ping = cap_net_raw+ep
https://forum.proxmox.com/threads/no-ping-from-non-root-user-in-debian-buster-lxc.72366/
https://serverfault.com/questions/696281/ping-icmp-open-socket-operation-not-permitted-in-vserver/915346#915346