prefix paths for PR builds

.drone.yml

@@ -88,7 +88,7 @@ steps:
     image: node
     commands:
       - yarn install
-      - yarn build
+      - PATH_PREFIX=/PR/${DRONE_PULL_REQUEST} yarn build --prefix-paths
       - du -sh public
 
   - name: rsync

gatsby-config.js

@@ -1,4 +1,5 @@
 module.exports = {
+  pathPrefix: process.env.PATH_PREFIX,
   siteMetadata: {
     title: "Chuck Dries",
     siteUrl: "https://chuckdries.com",