Add Alpine 3.15.0 to the alpine versions

[kelcecil]

This adds Alpine 3.15.0 to the built Alpine versions. Alpine 3.15 was released as stable on November 24th, and it includes the updated LTS version for NodeJS (v16).

I noticed in #99 a reference to version smoke tests, but I don't quite see how to run them. I'd be happy to run and triage issues out of the smoke test if I could get a nudge in the right direction.

[ericmj]

I noticed in #99 a reference to version smoke tests, but I don't quite see how to run them. I'd be happy to run and triage issues out of the smoke test if I could get a nudge in the right direction.

There are no automated smoke tests, but you can build the images on the oldest and newest support OTP versions, run them and see if application:start(ssl). works.

[hoyon]

I've built images for alpine 3.15.0 with erlang 23.2.2 and erlang 24.2 and they both build fine and running application:ensure_all_started(ssl). gives {ok,[crypto,asn1,public_key,ssl]} for both.

[ericmj]

Thanks for checking the builds. I will deploy this later today or tomorrow.