Hmm...
That worked:
1. In /etc/fstab on the host system
/etc/pve/nodes/ocs /certs fuse.bindfs perms=0000:u+rD,uid-offset=100000,gid-offset=100000 0 0
And then just:
pct set 103 -mp0 /certs,mp=/certs
And it works :D, looks like a hack but I am happy with that solution :).
BTW I am not sure how...