seems to work after I

1) installed Docker itself (needs Go language from Google .. quite a beast! filled 1Gb for its own incremental rebuilds)

2) installed docker-cli from Slackbuilds

3) ran /etc/rc.d/rc docker start (different command on systemd distros)

4) ran

mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc

(may be already done by your startup scripts)

5) ran docker run --rm --privileged multiarch/qemu-user-static --reset -p yes -c yes

6) ran test line

root@slax:~/src/appimagetool# docker run --rm -t --privileged arm64v8/ubuntu uname -m              WARNING: The requested image's platform (linux/arm64/v8) does not match the detected host platform (linux/386) and no specific platform was requested                                                 

aarch64                                                                                    root@slax:~/src/appimagetool#

for some reason it was not working as root without "privileged"!

But hopefully now I can.download few more linuxes for  exotic arches and look what their objcopy output looks like! So, hopefully more arches in our strange image-embedding code soon ;)

Appimagetool build also worked after I added "privileged" to its script!

env ARCH=x86_64  ci/build-in-docker.sh