fix image url in imageFunction (netlify#159)

Author: lindsaylevine

lib/templates/imageFunction.js

@@ -7,7 +7,7 @@ exports.handler = async (event) => {
  const quality = parseInt(q);
 
  const imageUrl = url.startsWith("/")
- ? `${process.env.URL || "http://localhost:8888"}${url}`
+ ? `${process.env.DEPLOY_URL || `http://${event.headers.host}`}${url}`
  : url;
  const image = await jimp.read(imageUrl);