docker: perform basic binfmt_misc validation in docker.py

docker: perform basic binfmt_misc validation in docker.py

Setting up binfmt_misc is outside of the scope of the docker.py script
but we can at least validate it with any given executable so we have a
more useful error message than the sed line of deboostrap failing
cryptically.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reported-by: Richard Henderson <richard.henderson@linaro.org>