In order for the LXC container to have full access the proxmox host directory, a subgid is set as owner of a host directory, and an ACL is used to ensure permissions.
Add the following line to /etc/pve/lxc/<CT_ID>.conf
mp0:/mount/point/on/host,mp=/mount/point/on/lxc
OR
pct set 100 -mp0 /mnt/bindmounts/shared,mp=/shared