This is not the official Alpine image, it is Glider Labs one.
The official image was also affected, too, it appears.
$ docker run -it alpine head -1 /etc/shadow
root:!::0::::: » docker run --rm -ti alpine:3.8 sh -c 'cat /etc/shadow | grep root'
root:!::0:::::
Also, has anyone reported this to the official Alpine repository? (since the Talos disclosure seems to be confused; it says official but has URLs to the Glider Labs version?)
Edit: Ah, so here's the relevant GitHub issue for official Alpine Linux docker: https://github.com/docker-library/official-images/pull/5516
<=3.5 is no longer supported. Everything newer is patched.