docker build -t spleeter:aarch64 .
Based on https://hub.docker.com/r/arm64v8/python/
Thanks to the discussions here: deezer/spleeter#696, I've found out the packages/dependencies needed to be installed/built for aarch64.
docker run --name spleeter_worker --rm -v $(pwd):/output spleeter:aarch64 spleeter separate -p spleeter:2stems -o output [file1] [file2]