[Container VPS ] Ubuntu 16.04 upgrade to Ubuntu 18.04

This guide is prepared for Container VPS services, that that comes with OpenVZ 7 virtualization. As these services come with 3.x kernel version, it is possible to upgrade the Ubuntu 16.04 version to Ubuntu 18.04. The recommendation is to make the upgrade on a fresh Ubuntu 16.04 OS installation, as the more customizations are done, the higher is the change for failure.

Warning:
Before making any changes, back up any required information from the VPS.

1. Check if your service has the required kernel version:
   uname -r
   The output should show Linux 3.x kernel version.

2. Update the current packages:
   apt update && apt upgrade

3. Install update-manager-core tool:
   apt install update-manager-core

4. Start the Upgrade:
   do-release-upgrade

5. Agree with the first request of enabling alternate SSH ports. Afterward, proceed with the upgrade details based on your preferences. You will be requested to enter a network interface name for IPv6 usage. The interface is "venet0". If at the end of configuration you will not agree with automatic reboot, you will have to reboot on the first possibility.

6. After you have finished, check the version:
   lsb_release -a
   This should show the following output:

   Distributor ID: Ubuntu
   Description: Ubuntu 18.04.4 LTS
   Release: 18.04
   Codename: bionic