Docker runs x86 images with qemu emulation which doesn’t always work on the M1. That segfault is somewhat of a known issue, but with the images being ran rather than Docker or flyctl. The best options are building with a docker daemon running on x86 system, like our remote builders or another docker host you manage.
It’s often intermittent, so maybe building with docker might help: docker build --platform linux/amd64
We’re contemplating doing remote builds by default on M1 Macs until these issues are fixed.