tests/docker/docker.py: support --include-executable

tests/docker/docker.py: support --include-executable

When passed the path to a binary we copy it and any linked libraries (if
it is dynamically linked) into the docker build context. These can then
be included by a dockerfile with the line:

  # Copy all of context into container
  ADD . /

This is mainly intended for setting up foreign architecture docker
images which use qemu-$arch to do cross-architecture linux-user
execution. It also relies on the host and guest file-system following
reasonable multi-arch layouts so the copied libraries don't clash with
the guest ones.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1468934445-32183-3-git-send-email-famz@redhat.com
Signed-off-by: Fam Zheng <famz@redhat.com>