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