Add overlayfs graph backend

This backend uses the overlayfs union filesystem for containers
plus hard link file sharing for images.

Each container/image can have a "root" subdirectory which is a plain
filesystem hierarchy, or they can use overlayfs.

If they use overlayfs there is a "upper" directory and a "lower-id"
file, as well as "merged" and "work" directories. The "upper"
directory has the upper layer of the overlay, and "lower-id" contains
the id of the parent whose "root" directory shall be used as the lower
layer in the overlay. The overlay itself is mounted in the "merged"
directory, and the "work" dir is needed for overlayfs to work.

When a overlay layer is created there are two cases, either the
parent has a "root" dir, then we start out with a empty "upper"
directory overlaid on the parents root. This is typically the
case with the init layer of a container which is based on an image.
If there is no "root" in the parent, we inherit the lower-id from
the parent and start by making a copy if the parents "upper" dir.
This is typically the case for a container layer which copies
its parent -init upper layer.

Additionally we also have a custom implementation of ApplyLayer
which makes a recursive copy of the parent "root" layer using
hardlinks to share file data, and then applies the layer on top
of that. This means all chile images share file (but not directory)
data with the parent.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Update CONTRIBUTING to reflect new PR-docs process.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Updated cover tool import path.

Signed-off-by: Hans Rødtang <hansrodtang@gmail.com>
pkg/devicemapper: missed MAINTAINERS on split

Signed-off-by: Vincent Batts <vbatts@redhat.com>
Add the possibility of specifying a subnet for --insecure-registry

Signed-off-by: Tibor Vass <teabee89@gmail.com>
Rewrite documentation for insecure registries

Signed-off-by: Tibor Vass <teabee89@gmail.com>
Merge pull request #7619 from alexlarsson/overlayfs

Add overlayfs graph backend
Merge pull request #9165 from hansrodtang/master

Updated cover tool import path.
Merge pull request #9172 from vbatts/vbatts-dm_maintainer

pkg/devicemapper: missed MAINTAINERS on split
Merge pull request #9100 from tiborvass/insecure-registry-cidr

Add the possibility of specifying a subnet for --insecure-registry
Merge pull request #9116 from erikh/update_contributing

Update CONTRIBUTING to reflect new PR-docs process.