Added proposed Docker Community Guidelines

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
Update nodejs_web_app.md

The image centos:centos6.4 doesn't exist. Instead you have to choose between the official centos6 or centos7. Both images does not work together with de yum npm install. When choosing an centos6.5 from another distributor (tutum) it works.
integcli: add some more docker utils

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
integcli: use dockerCmdInDir in build tests

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Update nodejs_web_app.md

Updated to documentation to use the centos6 image instead of the unofficial tutum image of centos6.4
networking.md: Networking between containers works without ip_forward

Docker-DCO-1.1-Signed-off-by: Erik Inge Bolsø <knan@redpill-linpro.com> (github: knan-linpro)
Return error for bad content-type
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
 #1852 Remote API documentation incorrect regarding private registries

Issues addressed:

- Remote API doco refers to "registry" parameter which is not supported
- Neglects to mention "tag" parameter which is supported
- Offers no guidance on pushing to private registries

Changes made:

- As directed in review comments, `registry` removed from all api versions (1.0-1.14)
- Have added documentation for `tag` parameter and guidance on private registries for versions 1.10-1.14 (I have only validated this on .10)

Docker-DCO-1.1-Signed-off-by: Lachlan Coote <lachlan.t.coote@gmail.com> (github: lcoote)
Check content type along with content length
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Merge pull request #7385 from crosbymichael/return-content-type-error

Return error for bad content-type
Accept vanilla Linux Foundation signoffs

This allows signing off commits with `git commit -s`

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)

Cleaned up DCO regex
Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)
Update CONTRIBUTING.md and test new DCO signature in this commit

Signed-off-by: Tibor Vass <teabee89@gmail.com>
Merge pull request #7380 from tiborvass/merge-7368

Accept vanilla Linux Foundation signoffs (Closes 7368)
Purge the bits of pkg/system that moved to libcontainer/system

Signed-off-by: Andrew Page <admwiggin@gmail.com>
Fix example of relative WORKDIR

Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)
Merge pull request #7363 from lcoote/master

 #1852 Remote API documentation incorrect regarding private registries
Merge pull request #7391 from LK4D4/fix_relative_workdir_example

Fix example of relative WORKDIR
Remove redundant checks in runconfig.Merge

Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)
Cleanup Cmd on ENTRYPOINT instruction

Fixes #5147
Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)
Inherit Cmd only if no --entrypoint specified on run

Fixes #5147
Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)