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
In the default Proxmox configuration, unpriviliged container subgids will have the prefix "10" followed by the expected 4-digit gid.