Add Dockerfile

[MacroPower]

Adds a very simple dockerfile. Users can mount torrent/music directories and supply extra arguments to configure it.

Also, the workdir is set to /musicscan/run, so that you can mount it somewhere easily to keep the log and state files.

Example usage:

docker run -d --name='musicscan' \
  -v '/mnt/user/music/':'/music':'ro' \
  -v '/mnt/user/torrents':'/torrents':'rw' \
  -v '/mnt/user/appdata/musicscan':'/musicscan/run':'rw' \
    'macropower/musicscan:0.1.2' \
      -u <username> \
      -p <password> \
      --loglevel debug \
      --output /torrents \
      /music