tests/docker: promote debian-riscv64-cross to a full image

tests/docker: promote debian-riscv64-cross to a full image

To be able to cross build QEMU itself we need to include a few more
libraries. These are only available in Debian's unstable ports repo
for now so we need to base the riscv64 image on sid with the the
minimal libs needed to build QEMU (glib/pixman).

The result works but is not as clean as using build-dep to bring in
more dependencies. However sid is by definition a shifting pile of
sand and by keeping the list of libs minimal we reduce the chance of
having an image we can't build. It's good enough for a basic cross
build testing of TCG.

Cc: "Daniel P. Berrangé" <berrange@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210914185830.1378771-2-richard.henderson@linaro.org>
[AJB: tweak allow_failure]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210917162332.3511179-5-alex.bennee@linaro.org>