chpasswd fails starting from Alpine 3.6

I’m able to use `chpasswd` to set user password until Alpine 3.5. Starting from Alpine 3.6 I get `PAM: Authentication failure` when I run the command.

To easily reproduce the issue:

$ docker run —rm -it alpine sh
/ # apk add shadow
/ # /usr/sbin/useradd -m -u 1000 new_user
/ # echo “new_user:mypassword” | chpasswd
Password: chpasswd: PAM: Authentication failure

(from redmine: issue id 10209, created on 2019-04-08, closed on 2019-06-19)

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