fix image url in imageFunction (#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);