Add key pid with help text into the schema.

schema.yml

@@ -116,6 +116,13 @@ mapping:
         desc: >
           Networking mode. Use the same values as the docker client --net parameter.
         required: false
+      "pid":
+        name: Pid
+        desc: >
+          Sets the PID mode to the host PID mode. This turns on sharing between container and the host operating system
+          the PID address space. Containers launched with this flag will be able to access and manipulate other
+          containers in the bare-metal machine's namespace and vise-versa.
+        required: false
       "dns":
         name: DNS
         desc: >