Make LXC containers directly accessable with ipv6

It seems to me that you are conflating a number of different things here. First, I doubt that the net mask on your server's ethernet port is actually /128. I suspect it's something else (/64 perhaps) and that you're on a shared segment with a bunch of other customers.

Judging by the output of your "ip -6 a" command:

4: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
    inet6 2607:5300:60:714::1/128 scope global 
       valid_lft forever preferred_lft forever
    inet6 fe80::ea40:f2ff:feed:106f/64 scope link 
       valid_lft forever preferred_lft forever
8: lxcbr0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 
    inet6 2607:5300:60:714::2/128 scope global 
       valid_lft forever preferred_lft forever
    inet6 fe80::b07b:e3ff:fe33:22e7/64 scope link 
       valid_lft forever preferred_lft forever
18: vethPVJQ6M: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
    inet6 fe80::fcb7:57ff:fe3c:bcd1/64 scope link 
       valid_lft forever preferred_lft forever

I would say that the /128 on the interfaces is an error. Your prefix appears to be 2607:5300:60:714::/64 (most likely).

Assuming that's correct, then you'll need to set up your interfaces file as follows (add your IPv4 as needed):

auto lxcbr0
iface lxcbr0 inet6 static
  bridge_ports eth0
  bridge_fd 0
  address 2607:5300:60:714::1
  net mask 64
  gateway 2607:5300:60:7ff:ff:ff:ff:ff

Note: It's not clear how you reach 2607:5300:60:7ff::/64 to get to your default gateway. It would be very useful to know how your provider expects you to configure your network or to have a first-hand look at any documentation they provided. Best guess from here is that the 2607:5300:60:714::/64 network is on the same link as 2607:5300:60:7ff::/64. That 2607:5300:60:7ff::/64 is used for the provider's infrastructure. It's unclear whether you get the entire 2607:5300:60:714:/64 or whether that's shared with other customers on the same link.

Assuming you have the freedom to assign addresses from within that range, then all you really need to do is connect your containers to the same lxcbr0 interface and assign each container's address to that bridge interface.

Again, this is just a best guess based on the data you provided. Without knowing your provider's actual configuration, it's impossible to tell for sure.