tests/docker: update and flatten debian-toolchain

Update to the latest stable Debian. While we are at it flatten into a
single dockerfile as we do not need anything from the base image to
build the toolchain. This is used to build both the nios and
microblaze toolchains.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220914155950.804707-29-alex.bennee@linaro.org>