add note about default socket to macos installation instructions

[nigelwtf]

This note should resolve #462 - and basically addresses the use case where MacOS users don't have the default socket option enabled in Docker Desktop - causing the (somewhat ambiguous) error Error response from daemon: pull access denied

Edit
and also resolve #453

[joaocasarin]

This shouldn't resolve #462. /var/run/docker.sock is available as following:
docker.sock -> /Users/joaocasarin/.docker/run/docker.sock

Even with that, it still doesn't work.