I would be very careful doing this. The minor revisions seem to come out quite often, and unless you check manually, you could miss a step.
I suppose you could parse Index of /alpine/ and decide what to do, but doing a blind ‘update’ will stop working when your subrevision is no longer current and/or there are no security patches for it.
Just keep in mind that it’s not the same as other distributions that tend to autosync subrevisions - which most of them will do. It doesn’t attempt to. That’s all up to you.