message: [docker] add build status badges
diff: [![Build Status][otbr-travis-svg]][otbr-travis]
  [![Coverage Status][otbr-codecov-svg]][otbr-codecov]
  +[![Docker Status][otbr-docker-amd64-linux-svg]][otbr-docker-amd64-linux]
  +[![Docker Status][otbr-docker-arm32v7l-raspbian-svg]][otbr-docker-arm32v7l-raspbian]
  ---
  @@ -34,6 +36,10 @@ More information about Thread can be found at [threadgroup.org](http://threadgro
  [otbr-travis-svg]: https://travis-ci.org/openthread/ot-br-posix.svg?branch=master
  [otbr-codecov]: https://codecov.io/gh/openthread/ot-br-posix
  [otbr-codecov-svg]: https://codecov.io/gh/openthread/ot-br-posix/branch/master/graph/badge.svg
  +[otbr-docker-amd64-linux]: https://hub.docker.com/r/openthread/otbr_amd64_linux
  +[otbr-docker-amd64-linux-svg]: https://img.shields.io/docker/cloud/build/openthread/otbr_amd64_linux.svg?label=docker%20%7C%20amd64_linux
  +[otbr-docker-arm32v7l-raspbian]: https://hub.docker.com/r/openthread/arm32v7l-raspbian
  +[otbr-docker-arm32v7l-raspbian-svg]: https://img.shields.io/docker/cloud/build/openthread/otbr_arm32v7l_raspbian.svg?label=docker%20%7C%20arm32v7l_raspbian
  ## Getting started
  