add note about default socket to macos installation instructions

[nigelwtf]

This note should resolve https://github.com/wagoodman/dive/issues/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 https://github.com/wagoodman/dive/issues/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.