docker: reduce noise when building travis.docker

Set the DEBIAN_FRONTEND and locale env vars to stop apt complaining so
much as we build the image.
Suggested-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Message-Id: <20170725133425.436-7-alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NFam Zheng <famz@redhat.com>

tests/docker/dockerfiles/travis.docker

 FROM quay.io/travisci/travis-ruby
+ENV DEBIAN_FRONTEND noninteractive
+ENV LANG en_US.UTF-8
+ENV LC_ALL en_US.UTF-8
 RUN apt-get update
 RUN apt-get -y build-dep qemu
 RUN apt-get -y install device-tree-compiler python2.7 python-yaml dh-autoreconf gdb strace lsof net-tools