Debug why cross compilation is so slow on osx

run.sh is super slow.

It may just be because the filesystem in Docker for Mac is so slow.

It may be that we can trim down the binary size and then the rsync will be quicker too.

https://medium.com/@jondot/8-steps-for-troubleshooting-your-rust-build-times-2ffc965fd13e may be helpful, if we change the cross docker file to use nightly temporarily.