docker: docker.py make --no-cache skip checksum test

If you invoke with NOCACHE=1 we pass --no-cache in the argv to
docker.py but may still not force a rebuild if the dockerfile checksum
hasn't changed. By testing for its presence we can force builds
without having to manually remove the docker image.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20170725133425.436-5-alex.bennee@linaro.org>
Signed-off-by: Fam Zheng <famz@redhat.com>
