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

docker testing improvements

# gpg: Signature made Mon 10 Sep 2018 04:56:45 BST
# gpg:                using RSA key CA35624C6A9171C6
# 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: Clean dangling tarball files
  docker: Sort CentOS packages
  docker: Sort Ubuntu packages
  docker: Sort Fedora packages (i386-cross)
  docker: Sort Fedora packages

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