Create the backup

1. find sd card diskutil list
2. unmount sd card diskutil unmountDisk /dev/disk5
3. copy sd card to image sudo dd if=/dev/rdisk5 of=input.img bs=4m

Shrink the image

Source Drewsif/PiShrink

1. image in input folder
2. rename image to input.img
3. docker build -t pishrink-image .
4. docker run --privileged=true -v ./input:/usr/src/app pishrink-image