Allow the container to share the PID namespace with the host

We want to be able to use container without the PID namespace.  We basically
want containers that can manage the host os, which I call Super Privileged
Containers.  We eventually would like to get to the point where the only
namespace we use is the MNT namespace to bring the Apps userspace with it.

By eliminating the PID namespace we can get better communication between the
host and the clients and potentially tools like strace and gdb become easier
to use.  We also see tools like libvirtd running within a container telling
systemd to place a VM in a particular cgroup, we need to have communications of the PID.

I don't see us needing to share PID namespaces between containers, since this
is really what docker exec does.

So currently I see us just needing docker run --pid=host

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

docs/sources/reference/commandline/cli.md

@@ -1604,6 +1604,7 @@ removed before the image is removed.
                                    Both hostPort and containerPort can be specified as a range of ports. 
                                    When specifying ranges for both, the number of container ports in the range must match the number of host ports in the range. (e.g., `-p 1234-1236:1234-1236/tcp`)
                                    (use 'docker port' to see the actual mapping)
+      --pid=host		 'host': use the host PID namespace inside the container.  Note: the host mode gives the container full access to local system services such as D-bus and is therefore considered insecure.
       --privileged=false         Give extended privileges to this container
       --restart=""               Restart policy to apply when a container exits (no, on-failure[:max-retry], always)
       --rm=false                 Automatically remove the container when it exits (incompatible with -d)

docs/sources/reference/run.md

@@ -133,11 +133,31 @@ While not strictly a means of identifying a container, you can specify a version
 image you'd like to run the container with by adding `image[:tag]` to the command. For
 example, `docker run ubuntu:14.04`.
 
+## PID Settings
+    --pid=""  : Set the PID (Process) Namespace mode for the container,
+           'host': use the host's PID namespace inside the container
+By default, all containers have the PID namespace enabled.
+
+PID namespace provides separation of processes. The PID Namespace removes the
+view of the system processes, and allows process ids to be reused including
+pid 1.
+
+In certain cases you want your container to share the host's process namespace,
+basically allowing processes within the container to see all of the processes
+on the system.  For example, you could build a container with debugging tools
+like `strace` or `gdb`, but want to use these tools when debugging processes
+within the container.
+
+    $ sudo docker run --pid=host rhel7 strace -p 1234
+
+This command would allow you to use `strace` inside the container on pid 1234 on
+the host.
+
 ## IPC Settings
     --ipc=""  : Set the IPC mode for the container,
                                  'container:<name|id>': reuses another container's IPC namespace
                                  'host': use the host's IPC namespace inside the container
-By default, all containers have the IPC namespace enabled 
+By default, all containers have the IPC namespace enabled.
 
 IPC (POSIX/SysV IPC) namespace provides separation of named shared memory segments, semaphores and message queues.