tests/docker/docker.py: add update operation

tests/docker/docker.py: add update operation

This adds a new operation to the docker script to allow updating of
binaries in an existing container. This is because it would be
inefficient to re-build the whole container just for an update to the
QEMU binary.

To update the executable run:

    ./tests/docker/docker.py update \
        debian:armhf ./arm-linux-user/qemu-arm

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1468934445-32183-6-git-send-email-famz@redhat.com
Signed-off-by: Fam Zheng <famz@redhat.com>