tests/docker: add a docker-exec-copy-test

tests/docker: add a docker-exec-copy-test

This provides test machinery for checking the QEMU copying logic works
properly. It takes considerably less time to run than starting a
debootstrap only for it to fail later. I considered adding a remove
command to docker.py but figured that might be gold plating given the
relative size of the containers compared to the ones with actual stuff
in them.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210202134001.25738-7-alex.bennee@linaro.org>