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

# gpg: Signature made Fri 09 Sep 2016 05:54:35 BST
# gpg:                using RSA key 0xCA35624C6A9171C6
# gpg: Good signature from "Fam Zheng <famz@redhat.com>"
# gpg: WARNING: This key is not certified with a trusted signature!
# gpg:          There is no indication that the signature belongs to the owner.
# Primary key fingerprint: 5003 7CB7 9706 0F76 F021  AD56 CA35 624C 6A91 71C6

* remotes/famz/tags/docker-pull-request:
  docker: silence debootstrap when --quiet is given
  docker: build debootstrap after cloning
  docker: make sure debootstrap is at least 1.0.67
  docker: print warning if EXECUTABLE is not set when building debootstrap image
  docker: debian-bootstrap.pre: print helpful message if DEB_ARCH/DEB_TYPE unset
  docker: debian-bootstrap.pre: print error messages to stderr
  docker: avoid dependency on 'realpath' package
  docker.py: don't hang on large docker output
  docker: Add a glib2-2.22 image

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