Download tar ball from https://alpinelinux.org/downloads/ I used alpine-rpi-3.15.0_rc6-aarch64, armhf does also work

unzip completely to a folder

Edit config.txt

copy

kernel=boot/vmlinuz-rpi
initramfs boot/initramfs-rpi

to the [all] block, similar to the [pi3] block

example:

...
[all]
arm_64bit=1
include usercfg.txt
kernel=boot/vmlinuz-rpi
initramfs boot/initramfs-rpi

Use Raspberry Pi imager to prep the SD card with partitions, 1 bootable FAT partition is needed to do the install

After imaging is complete, eject and open the SD card again to replace all imaged files with the files preped above.

lbu commit -d to save config after setup-alpine