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>