Volume-based authorized_keys now working.

With this change, you can specify SSH keys via environment variables in
addition to manually writing them using an authorized_keys volume by
locally mounting the Docker instance's `$HOME/.ssh/authorized_keys.d`
directory and editing the `authorized_keys` file therein. We've also
contributed documentation, which will be revised next.
Style fixes for README.
Merge branch 'release/1.1.0'