tests/docker: add travis dockerfile

This target grabs the latest Travis containers from their repository at
quay.io and then installs QEMU's build dependencies. With this it is
possible to run on broadly the same setup as they have on travis-ci.org.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Message-Id: <20161011161625.9070-2-alex.bennee@linaro.org>
Signed-off-by: NFam Zheng <famz@redhat.com>

tests/docker/dockerfiles/travis.docker

+FROM quay.io/travisci/travis-ruby
+RUN apt-get update
+RUN apt-get -y build-dep qemu
+RUN apt-get -y build-dep device-tree-compiler
+RUN apt-get -y install python2.7 dh-autoreconf
+ENV FEATURES pyyaml