In the GNU coreutils version of chown, ":" is a valid argument
(the command will result in a no-op), but POSIX chown does not
consider that valid.
If the user who ran build-image-*.sh was root, SUDO_UID and SUDO_GID
would not be set and, if the version of chown installed on the system
did not allow passing just a ":" as argument, the script would fail.
Let's default the value of SUDO_UID and SUDO_GID to 0 just in case.
8e24a17d0d
