This question may be same as https://stackoverflow.com/questions/10220531/how-to-set-system-wide-umask. But That is for debian based. I followed the instructions but I couldn't find any file named as /etc/pam.d/common-session. Also I don't know how to check that pam module packages are available or not ?
This is how I find out to set through gcos field. Feel free to edit the answer.
apk add shadow echo "session optional pam_umask.so" >> /etc/pam.d/base-session-noninteractive adduser -D -u 1000 -G deploy -g "umask=0026" -s /bin/sh deploy su deploy umask # This will output 0026 i.e your newly set umask value