unix: switch to Ubuntu 20.04 in Dockerfile
unix: switch to Ubuntu 20.04 in Dockerfile

ubuntu:19.04 is not supported anymore and update repos are unavailable, so
build process on Linux just fails. Latest stable ubuntu:20.04 works fine.
Also, DEBIAN_FRONTEND=noninteractive env variable has been added to prevent
unneeded questions during installation of tzdata package.

Fixes golang/go#38724

Change-Id: I2d7730d83fc051de2879d4f97fc7942090d2574c
Reviewed-on: https://go-review.googlesource.com/c/sys/+/230577
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Reviewed-by: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys