Thanks all for your help! I tried nix build --system aarch64-linux --max-jobs 0 --builders 'nix-docker aarch64-linux' and it worked perfectly, allowing me to create docker images using flakes on nix-darwin 
I know I could put these flags in nix.conf to avoid having to write them out each time, but is there a way of specifying this per project (kind of like a local .nixrc file?)