I'm not sure whether this is LXC specific, or whether this is some specific VM setting I should have configured differently. From var/build.log:

[sudo] password for ubuntu: Sorry, try again.
[sudo] password for ubuntu: 
sudo: 1 incorrect password attempt

This is due to the following, introduced in #8188

echo "ubuntu" | sudo -S true
sudo mkdir -p /usr/include/i386-linux-gnu/
sudo ln -s /usr/include/x86_64-linux-gnu/asm /usr/include/i386-linux-gnu/asm

I don't think gitian base image setup ever gave me the chance to configure a password. Looking at /etc/shadow confirms this, there isn't one:

ubuntu:x:1000:1000:ubuntu,,,:/home/ubuntu:/bin/bash

I think we need to find an alternative solution to this.

Possibly a change to gitian descriptors: a preparation script that is executed as root? This works fine:

LXC_ARCH=amd64 LXC_SUITE=trusty libexec/on-target -u root
root@gitian:~#