tests/docker: Use --arch-only when building Debian cross image

tests/docker: Use --arch-only when building Debian cross image

When building a Docker image based on debian10.docker on
a non-x86 host, we get:

 [2/4] RUN apt update &&     DEBIAN_FRONTEND=noninteractive eatmydata     apt build-dep -yy qemu
 Reading package lists... Done
 Building dependency tree
 Reading state information... Done
 Some packages could not be installed. This may mean that you have
 requested an impossible situation or if you are using the unstable
 distribution that some required packages have not yet been created
 or been moved out of Incoming.
 The following information may help to resolve the situation:

 The following packages have unmet dependencies:
  builddeps:qemu : Depends: gcc-s390x-linux-gnu but it is not installable
                   Depends: gcc-alpha-linux-gnu but it is not installable
 E: Unable to correct problems, you have held broken packages.

Fix by using the --arch-only option suggested here:
https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1866032/comments/1

Suggested-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210223211115.2971565-1-f4bug@amsat.org>
Message-Id: <20210305092328.31792-10-alex.bennee@linaro.org>