tests/docker: replace fedora-i386 with debian-i686

tests/docker: replace fedora-i386 with debian-i686

Fedora is gradually killing off i386 packages in its repos, via a
death-by-1000-cuts process. Thus Debian looks like a better long
term bet for i686 build testing. It has the added advantage that
we can generate it via lcitool too.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20231107164109.1449014-1-berrange@redhat.com>
[AJB: tweak commit msg, set correct prefix]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231120150833.2552739-2-alex.bennee@linaro.org>