docker-images: remove alpine-3.12
Created by: keegancsmith
We now only use 3.14
Follow-up from https://github.com/sourcegraph/sourcegraph/pull/34508
Test Plan: CI
Created by: keegancsmith
We now only use 3.14
Follow-up from https://github.com/sourcegraph/sourcegraph/pull/34508
Test Plan: CI