How to sync files between the workstation and EC2 without having to log the EC2 in manually every time

I want to do something that seems dead simple, but none of the options I've found are quite right (e.g. Dropbox).

The question is: what cloud sync service can I use to sync a folder on my workstation with the filesystem in an EC2 instance? Note these requirements:

• It must have a unattended/scriptable installation and configuration that happens on init of the EC2 (since EC2 instances are ephemeral)
• And thus it may only depend on EC2 environment variables for any service installation credentials
• The service on EC2 needs read-only, recursive synchronization (not plain downloading; there are too many files to simply download a directory archive and expand it periodically).
• Both workstation and EC2 are syncing with a shared source cloud repository like Dropbox, since that workstation is not always on/publicly accessible
• The app on my EC2 instance is nodeJS, for what it's worth!

The Dropbox Linux client, for example, (or nodejS libraries I've found) require attended installation, to visit a Dropbox URL every time the instance needs to log its Dropbox client in. Same is true for Bittorrent sync, requiring visiting a localhost URL to link with devices.

Even if another tiny EC2 instance is to sync Dropbox for example with Elastic File System. It might be longer-lived, but is still ephemeral and needs an unattended init-script installation.

Thanks in advance.