https://blog.sandstorm.io/news/2014-07-25-sharelatex.html
Most of our existing apps are actually ports of existing open source Linux-based servers. Most were ported by Jason Paryani over the last month, which tells you how easy it is. This includes apps written in PHP, Python, Node, etc.
Note that the dev tools (and all the platform source code) are available to anyone. There are some WIP docs on the wiki:
https://github.com/sandstorm-io/sandstorm/wiki
It's probably actually harder to port _away_ from Sandstorm simply because you have to add a login system and other such features. Adding is a lot harder than removing. :)