Docker:
* Provides a command line interface to manage containers (containers being, essentially, linux kernel namespaces, groups and a union file system, all brought together).
* Each app needs to provide its own auth.
* You need some orchestration in place to deploy and manage your apps.
* Ultimately, it requires _technical work_ to spin up applications.
Sandstorm:
* Provides a web interface for:
* Fetching/installing packaged apps.
* Exporting data, so you can trivially move hosts.
* Unified authentication: you log in once, and then you can access any app.
* Apps only take up server resources while you have a tab open.
* _Uses_ containers behind the scenes, but that's not the selling point.
A metaphor: you can think of Docker as being a bare bones OS, and Sandstorm as being that OS _plus_ a unified app store. As a non-technical software user, that app store makes the OS accessible to me - with a couple taps, I can have all sorts of new tools installed and ready to be used, all without ever considering memory models, network firewall rules, command line invocations, etc.
So, if you're a writing software and you need the flexibility of orchestrating containers, firewalls, auth, etc, then Docker is what you want. If instead, you just want to be able to edit documents, send emails, read your RSS feeds, etc, then you want to click a couple buttons and have your apps ready to go - you don't really care _how_ it's implemented, you just want your apps.