Install sparse in Ubuntu image

We don't need to build sparse from source, but we *do* need to install
it, 🤦

Fixes: 0fefa31 ("No longer need to build sparse from source")

Author: mpe

build/ubuntu/Dockerfile

@@ -39,6 +39,7 @@ RUN apt-get -q -y update && \
       u-boot-tools \
       rename \
       rsync \
+      sparse \
       xz-utils \
       $(/tmp/packages.sh) && \
     rm -rf /var/lib/apt/lists/* /tmp/packages.sh /var/cache/* /var/log/dpkg.log