Pass extra flags to docker runtime

[rossjones]

There are occassions where we want to run docker in privileged mode (--privileged) or to set the PID namespace (--pid with "host" or "container:ID") to enable new behaviours. There are security implications to these flags and so some thought should be put into how to enable this safely. Is this a deployment setting, or is it a deployment setting that allows the flags to work, or something else.

The values (once determined) need to added to the PidMode and Privileged keys of the HostConfig struct.