Add build script for docker

Useful docker build script from the jussi repo. It integrates the source commit and the docker tag into the /health status of hivemind. Without that, those variables show up as empty strings.

Result:

curl http://hivemind.url/health

{
  "state": {
    "db_head_block": 42459622,
    "db_head_time": "2020-04-11 22:53:30",
    "db_head_age": 9
  },
  "result": "head block age is 9, head block num is 42459622",
  "status": "OK",
  "sync_service": false,
  "source_commit": "0c851a07639ea842a313689a6b1307aeef9597df",
  "schema_hash": "adb5cd9b",
  "docker_tag": "autoclave",
  "timestamp": "2020-04-11T22:53:39.254393"
}