Propose testing and nightly version of the addon

[FraBoCH]

It would be helpful to test on hassio "beta" version, from the :testing or :nightly docker image published.
See: https://developers.home-assistant.io/docs/add-ons/presentation#offering-stable-and-canary-version

[FraBoCH]

Tested successfully for testing image on a fork: https://github.com/FraBoCH/hassio-addon/tree/testing
If a contributor can create a "testing" branch, I can create a pull request for that (very simple) change in config.
The same idea could be then done on nightly.

[goebelmeier]

That's nice, but won't help. We would need to set the :nightly tag for that branch, but as a result the add-on won't get updated. Home assistant supervisor doesn't compare the containers SHA digest against the digest in the container registry. It simply compares the installed container tag against the tag configured in the add-on repository config. And if there's any diff, it will suggest updating the addon.
That being said, we would require to generate a new tag for every nightly build, which would result in a huge tag sprawl at the container registry.
We already talked with HA maintainer Frenck, but they are not willing to change the current update check behavior in the near future.

[FraBoCH]

I see. For nightly it's definitively not manageable.
Maybe this principle could still be used for pre-release beta testing when that's useful (I had this idea when testing a fix for evcc-io/evcc#13007).