New docker image - able to grab a new AirConnect release upon startup

[MitchellSingleton]

Thank you, for creating this amazing software bridge! I have been running it for a long time on a standalone raspberry pi connecting to Sonos speakers. Only recently did I attempt to move to running AirConnect within a docker image.

The existing docker images never met the promise of keeping up with your releases, so i dove in and built my own image:
github - https://github.com/MitchellSingleton/docker-airconnect
docker - https://hub.docker.com/r/mitchellsingleton/docker-airconnect

The download of your release (via github api using tags) occurs during the container start. This provides the benefit of the container image not needing to be rebuilt / updated and only requires a stop and start.

I also provide a recommendation to use a persistant storage area, where the config and release downloads can be stored so that additional downloads are not needed when the file already exists in the persistant storage.

An additional benefit is that my image is built on an alpine base with s6 so the image size is only 11MB.

Please take a look and I would be much appreciative if you could add a link to my docker image.