[ecash-herald] Add docker files for automated prod deployments

Summary:
Adding docker files to be used by a teamcity build.

Note: secrets.js will need to be manually created on the eventual prod server

Test Plan:
Review files and assess if this is ok for teamcity deployment, in particular please advise on any preferred approaches for maintaining a secrets.js. We will end up using a similar approach for alias-server

To test it locally:

1. cp secrets.sample.js secrets.js
2. Update secrets.js 3.

docker build -t herald_local .
docker run --init --rm --name herald herald_local

and msgs send with your specified bot to your specified channel as blocks come in

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14331