qemu-build: Update base image to ubuntu:bionic

qemu 5.2 fails to build under Ubuntu 16.04. Upgrade to the base image
to the one specified by scripts/build-qemu-robot-docker.sh (bionic,
18.04).

Change-Id: Id135f0c3118a6aa997ddcd578fe98af4a9ae17de
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
1 file changed