Fix Dockerfile in Rails example

was missing libpq-dev

docs/rails.md

@@ -11,6 +11,7 @@ We're going to use Fig to set up and run a Rails/PostgreSQL app. Before starting
 Let's set up the three files that'll get us started. First, our app is going to be running inside a Docker container which contains all of its dependencies. We can define what goes inside that Docker container using a file called `Dockerfile`. It'll contain this to start with:
 
     FROM binaryphile/ruby:2.0.0-p247
+    RUN apt-get update -qq && apt-get install -y build-essential libpq-dev
     RUN mkdir /myapp
     WORKDIR /myapp
     ADD Gemfile /myapp/Gemfile