hs-airdrop: Add Dockerfile

[0xhaven]

Address #17
We've already an image on the namebase Docker Hub, but it would be nice if there were an official handshake-org build. Someone with access to the handshake-org GitHub account can add a Docker Hub integration that, and automatically builds and publishes an image on every update to master and maybe any update to https://github.com/handshake-org/hs-tree-data (though, that have to be manually triggered)

This is method is nicer because:

1. It doesn't require installing the right version of node/npm/etc
2. hs-airdrop is run in a completely un-networked container with read-only access to a single private key.

docker pull <namebase OR handshake-org>/hs-airdrop && docker run -it --network none -v ~/.ssh/<id_github>:/private_key:ro <namebase OR handshake-org>/hs-airdrop private_key <address> <fee>