Only add source after dependency download

This lets docker cache the ADD instruction, giving us a faster build.
Also, install docker-gen directly to /usr/local/bin.

Procfile

@@ -1,2 +1,2 @@
 nginx: nginx
-dockergen: /app/docker-gen -watch -only-exposed -notify "nginx -s reload" /app/nginx.tmpl /etc/nginx/sites-enabled/default
+dockergen: docker-gen -watch -only-exposed -notify "nginx -s reload" /app/nginx.tmpl /etc/nginx/sites-enabled/default