mnestor/nut-upsd repository overview

Based on https://hub.docker.com/r/instantlinux/nut-upsd⁠ for the most part. Redid the Dockerfile to use latest alpine.

Changes from instantlinux version:

1. BUILD_FROM = alpine:latest
2. Allow non-serial ups monitoring
3. Added NOTIFYCMD which should be populated from a config section

    environment:
      NOTIFYCMD: /usr/bin/nut_notify
    configs:
      - source: nut_notify
        target: /usr/bin/nut_notify