[faucet] Add a docker config for the faucet

Summary: Allow for easy build and deployment of the faucet. Shamelessly copied from the token-server docker file.

Test Plan:

docker build -t faucet -f faucet.Dockerfile .
docker run -p 18300:18300 -t faucet
curl http://127.0.0.1:18300/balance

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16979