Allow DB_URI to shortcut MONGODB_URI

In the FrozenRidge blog post that runs readers through setting up the StriderCD
Trusted Build, as well as in the configuration instructions in strider's README
(https://github.com/Strider-CD/strider#configuring), users are instructed to use
the DB_URI variable to set a custom mongodb:// connection string. The shell
script that starts strider, however, only looks for MONGODB_URI.