Execute a setup command on first clone

[sampsyo]

Builds have dependencies. Currently, we assume that the server just has all the dependencies installed a priori (i.e., the user will SSH in and install them manually). This is inconvenient: the Hooknook server should be able to configure itself as much as possible so you can just spin up a new, blank container and expect everything to work.

We could provide a second (configurable) command, perhaps defaulting to make deploysetup, that could would only be executed after the repository is first cloned.