tests/docker: Update the Ubuntu image to 19.04

This has aged a little and we have a separate LTS image for testing on
the older distros. Update it to a more recent release like its Fedora
cousin.

Besides it is useful to have something with gcc-9 on it for squashing
those stringop truncation errors.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
1 file changed