Make sure you know for sure which disk is the microSD. If you're not sure, don't follow these steps.. you could lose all the data on your Mac!

diskutil list
diskutil partitionDisk disk2 MBR FAT32 VOL1 256MB "Free Space" VOL2 R

export U=http://dl-cdn.alpinelinux.org/alpine/v3.8/releases/armhf
export F=alpine-rpi-3.8.4-armhf.tar.gz
curl $U/$F -o ~/Downloads/$F
curl $U/$F.sha256 -o ~/Downloads/$F.sha256
cd ~/Downloads/
shasum -c alpine-rpi-3.8.4-armhf.tar.gz.sha256

cd /Volumes/VOL1
tar xzvf ~/Downloads/alpine-rpi-3.8.4-armhf.tar.gz

cat>usercfg.txt<<EOF

On the Pi, the GPU and the CPU share RAM. This is a headless install, so give the GPU the least amount of RAM it can get by with (16MB). This also triggers the Pi to use a cutdown version of the firmware (start_cd.elf).

gpu_mem=16

Turn off audio and bluetooth. (Note "dt" stands for device tree.)

dtparam=audio=off,pi3-disable-bt

Enable mini UART as serial port (/dev/ttyS0).
Also, fixes VideoCore IV (aka the GPU or the VPU) frequency to 250MHz.

enable_uart=1
EOF

cd ~

unmount microSD and remove microSD card reader from the Mac

Shamelessy borrowed from http://markbucciarelli.com/posts/2019-05-05_install-alpine-on-raspberry-pi-using-macos.html