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 <tobias.klauser@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/linux/Dockerfile

diff --git a/unix/linux/Dockerfile b/unix/linux/Dockerfile
index 0cec8d7..9a42f2f 100644
--- a/unix/linux/Dockerfile
+++ b/unix/linux/Dockerfile
@@ -1,4 +1,7 @@
-FROM ubuntu:19.04
+FROM ubuntu:20.04
+
+# Disable interactive prompts on package installation
+ENV DEBIAN_FRONTEND noninteractive
 
 # Dependencies to get the git sources and go binaries
 RUN apt-get update && apt-get install -y  --no-install-recommends \