Support running inside docker

[ulope]

Since ethshadow has a whole bunch of dependencies it would be great if it were possible to run the simulation inside docker.

Shadow itself seems to support it.

I very briefly looked through the repo and the only issue I could spot seems to be that ethshadow uses docker to run some commands to generate genesis and validator keys (I think). Those would probably need to be made aware when running inside docker to fix the volume mount paths.

[ppopth]

That's great to have, but do you have any urgent or important use case for that? Otherwise, it seems that be a low-priority.