Rich container implementation

[allencloud]

Issue Description
Rich container is a very valuable feature of container technology. With the popular docker's concept, we should always run a single process or application in one single container. It seems to simplify process management in container. However it would affect application itself. This would be somewhat invasive to application which is definitely not a good way.

While rich container is one kind of container which could bear other system service running in container to help application run with whatever it wants. More specifically, we construct a system init process in the initialization of container.

Describe what happened:
Currently one single process or application in the only way suggested.

Describe what you expected to happen:
Rich container can be implemented in Pouch.

[allencloud]

I think this has been implemented in PR #767. Closing this.