Update documentation noting that SIGCHLD is not proxied.

Docker's --sig-proxy option sends all signals but one to a container. The
exception s SIGCHLD, which is deliberately ignored as it doesn't make sense to
send such a signal to a process in a container. Documentation updates will make
this less confusing if anyone does attempt to do this.

Docker-DCO-1.1-Signed-off-by: Matt Heon <mheon@redhat.com> (github: mheon)
Add a step to verify if Docker is working as expected

Like in the other installation guides I added a simple test
to verify that importing images and starting containers is
working.

Docker-DCO-1.1-Signed-off-by: Christian Berendt <berendt@b1-systems.de> (github: berendt)
add Docker version and branch to html header

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)
missing `sudo`
fix typo
Merge pull request #6751 from mchelen/patch-3

missing `sudo`
Merge pull request #6752 from mchelen/patch-5

fix typo
added linkage to tugboat, a scala client

Docker-DCO-1.1-Signed-off-by: Doug Tangren <d.tangren@gmail.com> (github: softprops)
Merge pull request #6747 from softprops/master

added linkage to tugboat, a scala client library
Remove extra "Docker" from docs index
Merge pull request #6761 from fgrehm/patch-1

Remove extra "Docker" from docs index
Merge pull request #6631 from SvenDowideit/add-versioning-to-docs-output

Add versioning to docs output
use the official  image for now - there's an opensuse image on its way

Docker-DCO-1.1-Signed-off-by: SvenDowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Merge pull request #6544 from mheon/sigproxy_docs

Update --sig-proxy documentation
Add the 1.13 API docs to the menu

Docker-DCO-1.1-Signed-off-by: SvenDowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Merge pull request #6768 from SvenDowideit/add-1.13-api-to-docs-menu

Add the 1.13 API docs to the menu
Merge pull request #6765 from SvenDowideit/pr_out_use_the_official_image_for_now_there_s_an_opensuse_image_on_its_way

use the official  image for now - there's an opensuse image on its way