tests/docker: add debian11 base image

We won't use this for building QEMU but we do need newer GCC's and
binutils for building some of our test cases.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200520140541.30256-10-alex.bennee@linaro.org>
2 files changed