Revert to Ubuntu 19.10

20.04 comes with Ruby 2.7, which emits some annoying deprecation warnigs that
can't be silenced even when using the RUBYOPT variables suggested for doing so.

Work around that for now by reverting to Ubuntu 19.10 which still uses 2.5 and
does not have this bug.

docker/rustembedded-osdev-utils/Dockerfile

@@ -2,7 +2,7 @@
 ##
 ## Copyright (c) 2017-2020 Andre Richter <andre.o.richter@gmail.com>
 ## Copyright (c) 2019-2020 Nao Taco <naotaco@gmail.com>
-FROM ubuntu:20.04
+FROM ubuntu:19.10
 
 ARG VCS_REF