In order to automatically mount CIFS on boot of an Alpine Linux instance, use a line like this in /etc/fstab:
//1.2.3.4/mydrive /mydrive cifs uid=1000,gid=1000,credentials=/root/.smb-credentials,iocharset=utf8,noperm 0 0
with a standard /root/.smb-credentials like this:
user=myuser password=raatahteiC1veiza8ahno8lu5quook
and run the following command to enable automount on boot:
rc-update add netmount boot
After a reboot, the filesystem should mount automatically.