docker-compose for testnet

[michielbdejong]

• [x] add letsencrypt
• [x] add Dockerfile
• [x] add docker-compose file for ilp-node + geth-rinkeby
• [ ] script that creates a Rinkeby wallet and saves it in the ilp-node config

[michielbdejong]

also the containers in the docker-compose don't get linked yet