Update INSTALL-digital-ocean.md

I've been hopping around on some newer providers and I often forget that Docker will (likely) never support running inside an OpenVZ container... in most cases. Often these providers run older kernels without AUFS support, and won't spin up a special kernel.

I'm not sure if this note is the "best" place to include the disclaimer; because this guide is very targeted. But it may save some upcoming support requests.