Added info re: image[:tag] to make docs consistent. Fixed a markdown issue.

Closes issue #6833

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Tianon made a micro-image (sub one meg?) that we can use as a very fast to download 'yes, your docker can download and run an image' test

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)
this info is out of date and forgotten

Docker-DCO-1.1-Signed-off-by: SvenDowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Update usingdocker.md

Change --help example command to match output in docs
Bugfix: only use io.Copy in hijack if attaching both stdout and stderr

Add regression tests to ensure issue is fixed.

Docker-DCO-1.1-Signed-off-by: Matt Heon <mheon@redhat.com> (github: mheon)
zsh: update zsh completion for docker command

zsh completion is updated with the content of
felixr/docker-zsh-completion.

Docker-DCO-1.1-Signed-off-by: Vincent Bernat <vincent@bernat.im> (github: vincentbernat)
Merge pull request #7088 from vincentbernat/fix/zsh-completion-update

Update zsh completion for docker command with upstream (felixr/docker-zsh-completion) latest
Merge pull request #7027 from SvenDowideit/use-a-hello-world-micro-image

Tianon made a micro-image (sub one meg?) that we can use as a very fast ...
Merge pull request #7072 from SvenDowideit/remove-infrastructure-dir

this info is out of date and forgotten
New screenshots and a few small edits.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Merge pull request #7068 from fredlf/add_image-tag

Added info re: image[:tag] to make docs consistent.
Merge pull request #7082 from bdevloed/patch-1

Update usingdocker.md
Merge pull request #7083 from mheon/6983_bugfix

Fix Panic with -t and -a stderr
Merge pull request #7094 from fredlf/update_docker-hub

New screenshots and a few small edits.