SOLVED but this problem may happen to others using OpenVZ containers.

- If you want to use your container with a static IP, you need to define it BEFORE acessing to the machine with "vzctl enter xxx"

- You can define the MAC address of eth0 within Proxmox GUI, but not what is needed to define the static IP

- Happily, you can define that from the proxmox host shell: just edit /etc/network/interfaces in /var/lib/vz/private/xxx and change the default dhcp config for eth0 as

auto eth0
iface eth0 inet static
        address (container IP address)
        netmask 255.255.255.255
        broadcast (container IP address)

        post-up route add (host / bridge access) dev eth0
        post-up route add default (host / bridge access)
        post-down route del (host / bridge access) dev eth0
        post-down route del default gw (host / bridge access)

- You can now start the machine from Proxmox and from proxmox shell type "vzctl enter xxx": you will run thru the TKL init script

- I was used to edit /etc/network/interfaces after vzctl enter: with a TKL template this fails miserably because TKL expects to have a complete internet access, which it has not!;