Create Dockerfile, containing the hashpreimage_cli executable

[HarryR]

We should make it easier to generate proofs for hashpreimage without needing to build the executable or install dependencies.

This should be a dockerfile that has hashpreimage_cli as the main entry point, but also includes the .so file and installs the ethsnarks python module.

This will help serve as a template for applications to be based on this repo (e.g. fork, then adapt to needs).

It should be as small as possible (e.g. Alpine Linux, with Python 3); however, for it to be small the development dependencies need to be uninstalled before packaging.

This needs to be setup on Docker Hub, with automatic builds and a brief self-test to verify it works as expected.