[FEAT] Upgrade to a more recent version of Ubuntu so we can utilize a more recent version of smartmontools package

[Kami]

Right now Docker images utilize Ubuntu 18.04. In Ubuntu 18.04 upstream apt repos, latest available pre-built version of smartmontool package is 7.0.

We should consider upgrading to Ubuntu 20.04 LTS or newer so we can utilize a more recent version of smartmontools package (e.g. 7.2-1).

smartmontools v7.2 is needed to be able to query some jmb39x based raid devices.

Another option would be to build latest version of smartmontools ourselves (that's the "workaround" I'm currently using in my custom Docker image), but long term it's probably better to upgrade to a more recent LTS version of Ubuntu since 18.04 will go EOL next year.

Related issues:

• SMART data for QNAP TR-004 function61/varasto#87
• Unable to run self test on QNAP TR-004 smartmontools/smartmontools#64