Introduce powerpipe container documentation

[omerosaienni]

Closes #23

[johnsmyth]

We have decided not to publish a container image because:

• The CLI is optimized for developer use on the command line.
• Everyone has specific goals and requirements for their containers.
• Container setup requires various mounts and access to configuration files.
• It's hard to support containers across many different environments.

We welcome users to create and share open-source container definitions for Powerpipe.