v20.05 branch: switch to Alpine 3.12 once enough binary pkgs are available
Alpine is currently building packages for their upcoming 3.12 release: http://dl-cdn.alpinelinux.org/alpine/v3.12/
3.12 has phosh in community, so we should switch our stable channel branch v20.05 from 3.11 to 3.12 as soon as the packages are built for all arches we support (x86_64, armhf, armv7, aarch64, x86).
By using phosh from Alpine's repositories, we can avoid forking all phosh related packages and the headaches that come with cross compiling the squeekboard rust package, which is part of phosh.