unit-test-docker: ubuntu:bionic -> ubuntu:disco

This updates us to the soon to be released version of ubuntu, skipping
cosmic so that we don't have to jump verisons again for a while. The
main draw here is to get a newer version of systemd to match the version
we are using in yocto (237 -> 240). This also gives us the benefit of
being able to clean up the gcc override for gcc-8 since it is the
default in disco.

Change-Id: I2327b481bdbe6b8a1c82d193bdc8d9b5e1a6e452
Signed-off-by: William A. Kennington III <wak@google.com>
2 files changed