Implement and document ENV-based configuration

Rainbow has a nice pattern where every config option can be set via cLI parameter OR env variable:

https://github.com/ipfs/rainbow/blob/d7b8111adf7d5cee590fe0a52ca8fb86a4bc297f/main.go#L115

It makes configuring runtimes like docker easier, and consistent across projects.

Someguy should do the same.