Add alpine support

[IAreKyleW00t]

Ideally, this repository/image should build and publish Alpine variants to match Caddy.

Much of the CI process is built with only 1 images in mind, so this will require a bit of rework to cleanly fit in. These would also need to be signed, and have their associated "shared tags" (v2, v2.1, etc.) updated.

[IAreKyleW00t]

Actually, the base image is alpine and that's all they publish. Their docs imply they're different, but they're actually the exact same. https://github.com/caddyserver/caddy-docker/blob/master/stackbrew-config.yaml#L10-L17

This repository could probably just add the -alpine tag variants to the list? I wanna do some more research first.