Poke backends through into static apps.

I've written this up to use a heroku-specific config file (`backends.yml`,
which is optional) rather than nginx config so that in principle we can move it
to implementations other than nginx.

bin/compile

@@ -14,6 +14,8 @@ nginx_tarball_url="https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/ngi
 mkdir -p $build_dir/bin
 curl -L $nginx_tarball_url | tar xzv -C $build_dir/bin
 nginx_version=$($build_dir/bin/nginx-$STACK -V 2>&1 | head -1 | awk '{ print $NF }')
+cp -a $bp_dir/scripts/boot $build_dir/bin/boot
+cp -a $bp_dir/scripts/config $build_dir/bin/config
 echo "-----> Installed ${nginx_version} to /app/bin"
 
 mkdir -p $build_dir/config

bin/release

@@ -5,5 +5,5 @@ cat <<EOF
 ---
 addons: []
 default_process_types:
-  web: erb config/nginx.conf.erb > config/nginx.conf && bin/nginx -p . -c config/nginx.conf
+  web: bin/boot
 EOF

config/nginx.conf.erb

@@ -25,9 +25,15 @@ http {
   client_body_timeout 5;
 
   server {
-    listen <%= ENV["PORT"] %>;
+    listen <%= port %>;
     server_name _;
     keepalive_timeout 5;
     root public/;
+
+<% backends.each do |backend| %>
+    location <%= backend['mount_point'] %> {
+      proxy_pass <%= backend['url'] %>;
+    }
+<% end %>
   }
 }

scripts/boot

+#!/bin/bash -e
+
+HERE=$(dirname "$0")
+
+"${HERE}/config/make-config" && "${HERE}/nginx" -p . -c config/nginx.conf
\ No newline at end of file

scripts/config/make-config

+#!/usr/bin/env ruby
+
+TEMPLATE = 'config/nginx.conf.erb'
+BACKENDS = 'backends.yml'
+CONFIG = 'config/nginx.conf'
+
+require 'erb'
+require 'yaml'
+
+def port
+  ENV['PORT'] || 5000
+end
+
+def backends
+  YAML.load(File.read(BACKENDS))
+rescue Errno::ENOENT
+  []
+end
+
+erb = ERB.new(File.read(TEMPLATE))
+
+File.write(CONFIG, erb.result(binding))