build.sh: Use Naohiro's $(whoami) default namespace

This way folks who aren't me don't need to explicitly set NAMESPACE.
Naohiro added the line in bdb9c6cb (Initial import, 2014-06-27, [1])
and then clarified the licensing in 8f90d189 (Add copyright notice:
issue #4, [2]).

Based-on-patch-by: Naohiro Aota <naota@gentoo.org>

[1]: https://github.com/naota/dockergentoo/commit/bdb9c6cbdaa8b3dc1579c6c4fd1ae5cd89db83cc
[2]: https://github.com/naota/dockergentoo/commit/8f90d189b7f4175f3447aa7f82b82799944f07c1
build.sh: Use 'id -u -n' instead of 'whoami' for POSIX compatibility

'whoami' isn't in the POSIX spec, but 'id -u -n' is in POSIX and does
the same thing [2].

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/idx/utilities.html
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/id.html#tag_20_59_18