Build Docker image

[plu]

Example usage via docker-compose.yml:

services:
  hetzner-dyndns:
    image: ghcr.io/plu/hetzner-api-dyndns:master
    environment:
      HETZNER_AUTH_API_TOKEN: ${HETZNER_API_KEY}
      HETZNER_RECORD_NAME: home
      HETZNER_ZONE_ID: ${HETZNER_ZONE_ID}
      UPDATE_INTERVAL: 60