fix: fix ssh agent forwarding on macOS

[lsorber]

Problem [1]:

1. We need to use a non-root user in the container so that files written in a mounted volume on Linux are owned by the host's user instead of root.
2. The Docker for Mac docs explain that you need to mount /run/host-services/ssh-auth.sock to forward the host's SSH agent.
3. Docker for Mac will mount that socket as root, which means that the non-root container user doesn't have permission to use the mounted SSH agent socket.

Solution: on startup, we chown the socket to the non-root user app.

[1] docker/for-mac#5303 (comment)