Required ports

[slifty]

I just faced an issue where I was running postgres already on my host machine for another project. Vagrant's requests were getting routed to that instance instead of to the docker / devenv instance of postgres.

In an ideal world our devenv would be able to function with a minimal port footprint on the host machine, though for now that may be challenging so long as we're using docker and vagrant (it MIGHT be achievable though, if we could somehow have vagrant talk directly to docker rather than going through host / 10.0.2.2).

In the short term, though, we should document the required port footprint in our README.

It would also be useful to have the docker compose command somehow flip out if various required ports are already in use on the host machine.