I have the same problem.
The container cannot mount the NFS export because the nfs kernel module is not loaded, and the container cannot load it.
Once the host as loaded the module (to access NFS storage for iso for exemple), the container can mount the NFS exports.
And then, if your restart it, they're mounted at boot...
What I have done is to add the nfs module in /etc/modules.
It will be loaded by the host at boot time.
I wonder why proxmox mounts the NFS storages only when you access it for the first time...