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)
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)
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.
Merge branch 'master' of github.com:dotcloud/docker into selinux-opts

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)