Both of your said "container" instead of "virtual machine" so I guess you are using LXC virtualization here? Mind that these containers are not dedicated machines by themselves but use the host's kernel.
The proxmox documentation explicitly states:
For security reasons, access to host resources needs to be restricted. Therefore, containers run in their own separate namespaces. Additionally some syscalls (user space requests to the Linux kernel) are not allowed within containers.
Further down, they suggest you could try
To disable AppArmor for a container, add the following line to the container configuration file located at
/etc/pve/lxc/CTID.conf:lxc.apparmor.profile = unconfinedWarning Please note that this is not recommended for production use.
and check if this makes time synchronization work but mind the warning. If so, the proxmox support forum (or even Google) may know how to give only the time setting capability selectively to your Pi-hole container. Unfortunately, the documentation isn't clear about if this also allows the aforementioned restricted syscalls so we really need to try it.
I'm not myself familiar with Proxmox, but it turns out I'm the end that proxmox won't allow your Pi-hole container to set the time for you automatically, you can easily disable this feature by seeing either the config option ntp.sync.interval = 0 or ntp.sync.server = "".