Merge remote-tracking branch 'remotes/famz/tags/docker-pull-request' into staging

# gpg: Signature made Fri 10 Nov 2017 13:41:01 GMT
# gpg:                using RSA key 0xCA35624C6A9171C6
# gpg: Good signature from "Fam Zheng <famz@redhat.com>"
# Primary key fingerprint: 5003 7CB7 9706 0F76 F021  AD56 CA35 624C 6A91 71C6

* remotes/famz/tags/docker-pull-request:
  docker: correctly escape $BACKEND in the help output
  docker: Improved image checksum

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>