Can be used to build 32bit (aka multilib) packages. With all the
simplification and de-duplication done with the previous commits, this
turns out to be a fairly simple job ;-)

Notes:
 - there is currently no test stage for this container
 - the release bits will come with next commit

v2:
 - preserve extra.conf for base/base-devel

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>